feat: constexpr support with trivial unions

[ChronoInferna]

Close #36 by implementing p3726 and p3074.

As of today, I don't think Clang has actually merged in support for those two papers, so I would appreciate some help verifying if these are valid changes or not as I can't test the new code (currently macro'd out) until that happens.

More specifically, I don't know which Clang version will actually release with trivial unions and as such might require changing the version in config.hpp. I also don't know what the API for std::start_lifetime actually is, as it's in p3726 but the standard library only has std::start_lifetime_as.

Otherwise here are the changes:

• Added detection of trivial union support via the __cpp_trivial_union macro and defined the BEMAN_INPLACE_VECTOR_HAS_TRIVIAL_UNION macro in config.hpp.
• Updated the satisfy_constexpr concept to always be true when trivial unions are supported, allowing more constexpr usage.
• Introduced union_storage struct for non-trivial types when trivial unions are available, enabling union-based storage and appropriate lifetime management.
• Modified the storage_for type alias to select union_storage when trivial unions are supported, otherwise falling back to the previous logic.
• Ensured that the old raw_byte_based_storage is only used when trivial unions are not available.
• Added appropriate tests

[coveralls]

coverage: 98.649%. remained the same — ChronoInferna:trivial-unions into bemanproject:main

[ednolan]

Tip: If these features exist in Clang trunk, you could add Clang trunk to the CI by specifying:

          "clang": [
            { "versions": ["trunk"],