is_invocable_r does not support function types

## Brief feature description

`is_invocable_r` and `is_invocable` do not work properly with function types (as opposed to function pointers) as in
`cxx::is_invocable_r`.

This is a minor issue since it works with function pointers such as `cxx::is_invocable_r` but is not compatible with STL.

This should be remedied with C++17 as there `std::is_invocable_r ` is available.

The test coverage for different types of callables is too low though and hence we need to add more tests for supported cases: function pointers, method pointers, lambdas (exist), functors (general case).

## Detailed information

Usage with function name types is optional for but more test cases for working types are needed.

Start by locating the implementations of is_invocable_r and is_invocable and the existing callable-type tests. Review coverage for function pointers, method pointers, lambdas, and functors, then verify that function-type usage either works as intended or is explicitly covered by the tests.

Written by the indexing model from the issue text.