Tests fail with gcc 7.0.1 due to new aliasing warnings

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Running `make test` with gcc 7.0.1 fails due to new aliasing warnings, promoted to errors by the `-Werror` switch in the makefile:

```
g++ -c -o out/binary_visitor_1.o test/t/binary_visitor_1.cpp -Iinclude -isystem test/include -std=c++11 -Werror -Wall -Wextra -pedantic -Wformat=2 -Wsign-conversion -Wshadow -Wunused-parameter -O3 -DNDEBUG -march=native -DSINGLE_THREADED -fvisibility-inlines-hidden -fvisibility=hidden -I/usr/include/catch -pthread
In file included from test/t/binary_visitor_1.cpp:2:0:
include/mapbox/variant.hpp: In instantiation of ‘T& mapbox::util::variant::get() [with T = int; typename std::enable_if<(mapbox::util::detail::direct_type::index != mapbox::util::detail::invalid_value)>::type* = 0; Types = {int, double}]’:
test/t/binary_visitor_impl.hpp:180:9: required from here
include/mapbox/variant.hpp:724:20: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
return *reinterpret_cast(&data);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/mapbox/variant.hpp: In instantiation of ‘T& mapbox::util::variant::get() [with T = double; typename std::enable_if<(mapbox::util::detail::direct_type::index != mapbox::util::detail::invalid_value)>::type* = 0; Types = {int, double}]’:
test/t/binary_visitor_impl.hpp:187:9: required from here
include/mapbox/variant.hpp:724:20: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
include/mapbox/variant.hpp: In instantiation of ‘const T& mapbox::util::variant::get_unchecked() const [with T = int; typename std::enable_if<(mapbox::util::detail::direct_type::index != mapbox::util::detail::invalid_value)>::type* = 0; Types = {int, double}]’:
include/mapbox/variant.hpp:472:51: required from ‘static R mapbox::util::detail::binary_dispatcher::apply_const(const V&, const V&, F&&) [with F = const add_visitor&; V = mapbox::util::variant; R = double; T = int; Types = {double}]’
include/mapbox/variant.hpp:893:73: required from ‘static decltype (mapbox::util::detail::binary_dispatcher::apply_const(v0, v1, forward(f))) mapbox::util::variant::binary_visit(const V&, const V&, F&&) [with F = const add_visitor&; V = mapbox::util::variant; R = double; Types = {int, double}; decltype (mapbox::util::detail::binary_dispatcher::apply_const(v0, v1, forward(f))) = double]’
include/mapbox/variant.hpp:987:27: required from ‘decltype (V:: binary_visit(v0, v1, forward(f))) mapbox::util::apply_visitor(F&&, const V&, const V&) [with F = const add_visitor&; V = mapbox::util::variant; decltype (V:: binary_visit(v0, v1, forward(f))) = double]’
test/t/binary_visitor_impl.hpp:28:5: required from here
include/mapbox/variant.hpp:737:16: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
return *reinterpret_cast(&data);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/mapbox/variant.hpp: In instantiation of ‘T& mapbox::util::variant::get_unchecked() [with T = int; typename std::enable_if<(mapbox::util::detail::direct_type::index != mapbox::util::detail::invalid_value)>::type* = 0; Types = {int, double}]’:
include/mapbox/variant.hpp:493:45: required from ‘static R mapbox::util::detail::binary_dispatcher::apply(V&, V&, F&&) [with F = const add_visitor&; V = mapbox::util::variant; R = double; T = int; Types = {double}]’
include/mapbox/variant.hpp:900:67: required from ‘static decltype (mapbox::util::detail::binary_dispatcher::apply(v0, v1, forward(f))) mapbox::util::variant::binary_visit(V&, V&, F&&) [with F = const add_visitor&; V = mapbox::util::variant; R = double; Types = {int, double}; decltype (mapbox::util::detail::binary_dispatcher::apply(v0, v1, forward(f))) = double]’
include/mapbox/variant.hpp:994:27: required from ‘decltype (V:: binary_visit(v0, v1, forward(f))) mapbox::util::apply_visitor(F&&, V&, V&) [with F = const add_visitor&; V = mapbox::util::variant; decltype (V:: binary_visit(v0, v1, forward(f))) = double]’
test/t/binary_visitor_impl.hpp:68:5: required from here
include/mapbox/variant.hpp:713:16: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
return *reinterpret_cast(&data);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/mapbox/variant.hpp: In instantiation of ‘const T& mapbox::util::variant::get_unchecked() const [with T = double; typename std::enable_if<(mapbox::util::detail::direct_type::index != mapbox::util::detail::invalid_value)>::type* = 0; Types = {int, double}]’:
include/mapbox/variant.hpp:401:44: required from ‘static R mapbox::util::detail::binary_dispatcher_rhs::apply_const(const V&, const V&, F&&) [with F = const add_visitor&; V = mapbox::util::variant; R = double; T0 = int; T1 = double]’
include/mapbox/variant.hpp:477:80: required from ‘static R mapbox::util::detail::binary_dispatcher::apply_const(const V&, const V&, F&&) [with F = const add_visitor&; V = mapbox::util::variant; R = double; T = int; Types = {double}]’
include/mapbox/variant.hpp:893:73: required from ‘static decltype (mapbox::util::detail::binary_dispatcher::apply_const(v0, v1, forward(f))) mapbox::util::variant::binary_visit(const V&, const V&, F&&) [with F = const add_visitor&; V = mapbox::util::variant; R = double; Types = {int, double}; decltype (mapbox::util::detail::binary_dispatcher::apply_const(v0, v1, forward(f))) = double]’
include/mapbox/variant.hpp:987:27: required from ‘decltype (V:: binary_visit(v0, v1, forward(f))) mapbox::util::apply_visitor(F&&, const V&, const V&) [with F = const add_visitor&; V = mapbox::util::variant; decltype (V:: binary_visit(v0, v1, forward(f))) = double]’
test/t/binary_visitor_impl.hpp:28:5: required from here
include/mapbox/variant.hpp:737:16: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
return *reinterpret_cast(&data);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/mapbox/variant.hpp: In instantiation of ‘T& mapbox::util::variant::get_unchecked() [with T = double; typename std::enable_if<(mapbox::util::detail::direct_type::index != mapbox::util::detail::invalid_value)>::type* = 0; Types = {int, double}]’:
include/mapbox/variant.hpp:407:38: required from ‘static R mapbox::util::detail::binary_dispatcher_rhs::apply(V&, V&, F&&) [with F = const add_visitor&; V = mapbox::util::variant; R = double; T0 = int; T1 = double]’
include/mapbox/variant.hpp:498:74: required from ‘static R mapbox::util::detail::binary_dispatcher::apply(V&, V&, F&&) [with F = const add_visitor&; V = mapbox::util::variant; R = double; T = int; Types = {double}]’
include/mapbox/variant.hpp:900:67: required from ‘static decltype (mapbox::util::detail::binary_dispatcher::apply(v0, v1, forward(f))) mapbox::util::variant::binary_visit(V&, V&, F&&) [with F = const add_visitor&; V = mapbox::util::variant; R = double; Types = {int, double}; decltype (mapbox::util::detail::binary_dispatcher::apply(v0, v1, forward(f))) = double]’
include/mapbox/variant.hpp:994:27: required from ‘decltype (V:: binary_visit(v0, v1, forward(f))) mapbox::util::apply_visitor(F&&, V&, V&) [with F = const add_visitor&; V = mapbox::util::variant; decltype (V:: binary_visit(v0, v1, forward(f))) = double]’
test/t/binary_visitor_impl.hpp:68:5: required from here
include/mapbox/variant.hpp:713:16: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
return *reinterpret_cast(&data);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors
make: *** [Makefile:100: out/binary_visitor_1.o] Error 1
```

Start with include/mapbox/variant.hpp, especially get() and get_unchecked(), which are reported by the failing test compilation. Run make test with gcc 7.0.1 and verify that the aliasing warnings no longer become errors and the test suite completes successfully.

Written by the indexing model from the issue text.