Implements parts of:
- P0898R3 Standard Library Concepts
- P1754 Rename concepts to standard_case for C++20, while we still can
Reviewed By: Mordante
Differential Revision: https://reviews.llvm.org/D98983
moves tests into directories matching their stable names so that the
tests can reflect the concept name
Differential Revision: https://reviews.llvm.org/D99104