Add implementation of std::is_callable

This is proposed for C++17 and helpful to checking correctness of predicates supplied to algorithms. This also commes with a test that checks is the behaviour is correct, especially with respect to various combinations of r-values, r-value references, and l-value-references.

I'll merge this small feature myself once the CI passed. But I'd also like to have this in 2.5.

Mentioned in commit 381de64f

Status changed to merged

Mentioned in commit 3a2e7d32

Mentioned in merge request !152 (merged)

Mentioned in commit 70ec21ea

mentioned in commit 42e343bc