Make pair trivially move constructible

On the develop branch hana::pair isn't is_trivially_move_constructible anymore! On the master branch it is.

```
#include
#include

int main(){
auto p = boost::hana::make_pair(0, 0);
static_assert(std::is_trivially_copy_constructible< decltype(p) >::value, "");
static_assert(std::is_trivially_move_constructible< decltype(p) >::value, ""); // Error
}
```

I tested with clang-TOT and GCC 7.1.1 (20170509)

Begin by running the supplied minimal program on the develop and master branches, then locate boost::hana::pair and compare its construction behavior. Done means the std::is_trivially_move_constructible assertion passes on the affected branch without regressing the copy-constructibility assertion.

Written by the indexing model from the issue text.