libs: stabilize ascii module

This is an initial API stabilization pass for `std::ascii`. Aside from
some renaming to match conversion conventions, and deprecations in favor
of using iterators directly, almost nothing is changed here. However,
the static case conversion tables that were previously public are now private.

The stabilization of the (rather large!) set of extension traits is left
to a follow-up pass, because we hope to land some more general machinery
that will provide the same functionality without custom traits.

[breaking-change]