vector_algorithms.cpp: cleanup

[AlexGuteniev]

Fix accumulated inconsistencies in vector_algorithms.cpp, separate commit each:

• More const, lately I've been trying to add const everywhere, but there is missing one in older code
• Use the same type for a variable as in the other similar branch. Unsigned type is selected, as we do bitwise thing with it
• Drop _CSTD, the C run time things used unqualified in this file in some other places
• _Avx2_tail_mask_32 now takes bytes count. It is odd from the perspective of the function itself, but almost every call site has bytes, and even for the ones that don't, the lack of multiplication by 4 and multiplication by 2 instead of by 8 look odd.

Deliberately skipped namespace as per #5418 for now. These look more controversial, so I'd do just clean stuff first.

[AlexGuteniev]

Added another commit with noexcept lambdas.
This is still consistency. noexcept isn't useful there.

[StephanTLavavej]

Thanks, these changes are great - especially the count-in-bytes and unsigned long mask changes! 😻 I pushed a syntax nitpick and a comment fix. STL SEES ALL.

[StephanTLavavej]

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

[StephanTLavavej]

🧹 🚀 😸