* use our own implementation of is_constant_evaluated() that defers to __builtin_is_constant_evaluated() for clang and gcc and returns false in all other cases.
* replace all occurences of `requires` with a comparable SFINAE `std::enable_if` template declaration.
* replace all uses of `std::span` with `base::span`.
* provide an implementation `BitCast` which is trivial implementation of `std::bit_cast`, that does a `memcpy` and static size and type checking and replace all uses of the standardized version.
* revert `span.h` to the version from before it used std::ranges throughout.