add <bit> with P0553R4 and P0556R3 (w/ rename from D1956)

[barcharcraz]

Description

Adds header with implementation of P0553R4 and P0556R3 (w/ rename from D1956).
Currently it's only lit up on clang and EDG, however the implementation will work with MSVC
pending an upcoming release.

This mirrors Microsoft internal MSVC-PR-213718

Checklist

Be sure you've read README.md and understand the scope of this repo.

If you're unsure about a box, leave it unchecked. A maintainer will help you.

• Identifiers in product code changes are properly _Ugly as per
  https://eel.is/c++draft/lex.name#3.1 or there are no product code changes.
• The STL builds successfully and all tests have passed (must be manually
  verified by an STL maintainer before automated testing is enabled on GitHub,
  leave this unchecked for initial submission).
• These changes introduce no known ABI breaks (adding members, renaming
  members, adding virtual functions, changing whether a type is an aggregate
  or trivially copyable, etc.).
• These changes were written from scratch using only this repository,
  the C++ Working Draft (including any cited standards), other WG21 papers
  (excluding reference implementations outside of proposed standard wording),
  and LWG issues as reference material. If they were derived from a project
  that's already listed in NOTICE.txt, that's fine, but please mention it.
  If they were derived from any other project (including Boost and libc++,
  which are not yet listed in NOTICE.txt), you must mention it here,
  so we can determine whether the license is compatible and what else needs
  to be done.

[CaseyCarter]

the implementation will work with msvc pending and upcoming release

"MSVC" ... "an" upcoming release

this mirrors Microsoft internal VSO-213718

"This". Also, VSO-213718 is .NET bug: our internal bugs and PRs have separate namespaces, this links to a bug rather than a pull request.

[CaseyCarter]

The code is fine, but the commit message needs a bit of cleanup.

[StephanTLavavej]

Congratulations!