P0784R7 Library Support For More constexpr Containers about stl

[miscco]

@MichaelRizkalla, I had a look at this because MSVC seems to now support __cpp_constexpr_dynamic_alloc and it is quite difficult to get it right with the huge machinery that std::vector uses

I fixed the merge conflicts and also some broken machinery / test code. You can find the fixes in my P0784R7 branch here https://github.com/miscco/STL/tree/P0784R7

[MichaelRizkalla]

@miscco, I am still trying to wrap my head around the entire code base and was expecting my PR to be simple to deal with 😿 . Is there any recommendation you see fit to handle this PR given the same content is implemented in your P0784R7 branch?

[miscco]

"My" branch is your branch with merge conflicts resolved and some fixes for MSVC, so you should be able to either cherry-pick, or rebase or reset on that.

Note that i did not fix anything else