This commit makes the user-facing StdLibExtras templates and utilities
arguably more nice-looking by removing the need to reach into the
wrapper structs generated by them to get the value/type needed.
The C++ standard library had to invent `_v` and `_t` variants (likely
because of backwards compat), but we don't need to cater to any codebase
except our own, so might as well have good things for free. :^)
The Coverity compiler doesn't support C++2a yet, and thus doesn't
even recognize concept keywords. To allow serenity to be built and
analyzed on such compilers, add a fallback underdef to perform
the same template restriction based on AK::EnableIf<..> meta
programming.
Note: Coverity does seem to (annoyingly) define __cpp_concepts, even
though it doesn't support them, so we need to further check for
__COVERITY__ explicitly.
