bit_array compile-time / construction-time containers

[PeterCDMcLean]

Summary of major changes:

Algorithms:

• to_from_string
• division, multiplication, addition by scalar
• accumulate, accumulate_while, accumulate_backward, accumulate_backward_while

Misc:

• mdspan accessors
• user defined literal
• bitsof
• several template/type utilities in bit_details
• general clean-up

Iterators:

• iterator word adapter
• consolidation of bit_pointer and bit_iterator
• bit_reference holds a true reference or a proxy reference

Containers:

• bit_array compile-time
• bit_array construction-time
• bit_array_ref
• bit_span

Compilers:

• MSVC support
• clang-cl