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. :^)
Add requires clauses to constraints on InputStream and OutputStream
operator<< / operator>>. Make the constraint on String::number a
requires clause instead of SFINAE. Also, fix some unecessary IsSame in
Trie where specialized traits exist for the given use cases.
