Implement P0448R4 spanstream

[miscco]

This implements spanstream that was recently voted into C++23

Fixes #1970

[StephanTLavavej]

Partial review (to be available as a video recording in the near future), I looked at everything except stl/inc/spanstream and P0448R4_spanstream/test.cpp.

[StephanTLavavej]

Video reviewed basic_spanbuf and the recent commits; basic_ispanstream and below, and the new test, remain. (This is a lot of code, thanks for working on it! 😻)

[StephanTLavavej]

Thanks! I'll validate and push changes.

[StephanTLavavej]

@miscco I pushed changes to use variable templates in the test as @barcharcraz suggested. It's slightly fewer lines of source code, and avoids emitting function calls (which isn't a big deal in test code, but can make stepping through a bit nicer). I also took this opportunity to rename get_input_array to input_ptr because "1 2 3 4 5" (in both the old and new code) decays from an array to a pointer.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo now. Please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for continuing to implement a stream of span features! 😹 🚀 🎉