make an independent implementation of `unique_resource`

[JeffGarland]

@robert-andrzejuk - unblocking this. I've implemented most of my suggestions and resolved comments. Also made current with current main branch. This will now hopefully pass all the CI

My current thinking is we should create a separate issue for the remaining items - including unique_resource. My personal development preference is to do small improvements and update main frequently (especially now that main is being mirrored in godbolt).

Originally posted by @JeffGarland in #20 (comment)

[davidhunter22]

It suggest in a few places that clang and GCC are supported. I believe this is only the case with clang if the GCC standard library libstdc++ is uses not the clang standard library libc++. I looked in the libc++ source tree and there is no <experimental/scope> file.
It may be good to document somewhere that only libstdc++ is supported at present