Modularize <functional>

[miscco]

This modularizes functional into a host of small header files and makes them usable internally

[wmaxey]

There are some failures on MSVC caused by the implicit/explicit constructors being enabled both at once.

C:\Users\Wes\Documents\workspace\libcudacxx\include\cuda\std\detail\libcxx\include\__functional\../__utility/pair.h(208): error C2535: 'cuda::std::__4::pair<_T1,_T2>::pair(const _T1 &,const _T2 &) noexcept(<expr>)': member function already defined or declared
C:\Users\Wes\Documents\workspace\libcudacxx\include\cuda\std\detail\libcxx\include\__functional\../__utility/pair.h(199): note: see declaration of 'cuda::std::__4::pair<_T1,_T2>::pair'
C:\Users\Wes\Documents\workspace\libcudacxx\include\cuda\std\detail\libcxx\include\__functional\../__utility/pair.h(439): note: see reference to class template instantiation 'cuda::std::__4::pair<_T1,_T2>' being compiled
C:\Users\Wes\Documents\workspace\libcudacxx\include\cuda\std\detail\libcxx\include\__functional\../__utility/pair.h(312): error C2535: 'cuda::std::__4::pair<_T1,_T2>::pair(_Tuple &&)': member function already defined or declared
C:\Users\Wes\Documents\workspace\libcudacxx\include\cuda\std\detail\libcxx\include\__functional\../__utility/pair.h(304): note: see declaration of 'cuda::std::__4::pair<_T1,_T2>::pair'