constexpr: ITSY_BITSY_CONSTEXPR_CXX20

[ThePhD]

There should be support for marking every single method -- including the allocator methods -- constexpr given C++20 gives us the (proven) technology to make a small bit vector that works at compile-time.

A mostly-constexpr but not C++-17-gated enabled vector is here, for those who have sponsored me at a suitable level and have the permissions to see it.