[libc++] Add __is_callable type trait and begin granularizing type_traits

`__is_callable` is required to ensure that the classic algorithms are only called with functions or functors. I also begin to granularize `<type_traits>`.

Reviewed By: ldionne, #libc

Spies: libcxx-commits, mgorny

Differential Revision: https://reviews.llvm.org/D123114