SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
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. :^)
Use TypeLists to add test for IsIntegral, IsFloatingPoint, IsVoid,
IsNullPointer, IsArithmetic, IsFundamental, and AddConst type traits.
More can "easily" be added once the TypeList and macro magic is squinted
at for long enough :).
