Add for_each_n.hpp

[al-mission-2016]

Hi, Tristan. After P1243R4 ranges::for_each_n (as well as sample and clamp) was added into C++20. http://open-std.org/JTC1/SC22/WG21/docs/papers/2020/p1243r4.pdf http://eel.is/c++draft/alg.foreach

[tcbrindle]

Hi, thanks for the PR!

A couple of comments/requested changes:

• Since this is only one small function, I'm happy for it to be added to for_each.hpp -- it doesn't need to go in its own header file
• for_each_n_result can just be a using alias for for_each_result -- you don't need to duplicate the whole class template (at some point I need to update all the *_result types to match the C++20 spec, but that's a separate PR)
• The impl() static function in various algorithms exist to avoid (potentially) having two sets of constraint checks if the range overload were simply to forward to the iter-sentinel overload. But since there is only a single overload of for_each_n, there is no need for an impl function -- the algorithm implementation can just go in operator().

See uninitialized_fill_n for an example of what I'm after.

Also, importantly, please add tests. Contrary to the above, these should go in their own test/algorithm/for_each_n.cpp source file. Range-V3 has a test file here which you can adapt for NanoRange (but please make sure to keep the copyright/licence notice intact, of course).

[tcbrindle]

Implemented in #96