This patch series reworks the implementation of the ranges::_Partial,
to reuse the functors created from _Bind_back, which allows it to
benefit from it's storage optimization.
The first two patches are refactoring patches done in pepration:
* 1/3 merges _Bind_front and _Bind_back binder into one templated
_Binder, making the later available in c++20
* 2/3 moves _Binder and required classes to separate bits/binders.h
header, so they can be included without brining functional
* 3/3 implement ranges::_Partial in terms of _Bind_back_t.
