gh-100926: Move ctype's pointers cache from _pointer_type_cache to StgInfo (GH-131282)

Deprecate _pointer_type_cache and calling POINTER on a string.

Co-authored-by: neonene <53406459+neonene@users.noreply.github.com>
Co-authored-by: Jun Komoda <45822440+junkmd@users.noreply.github.com>
Co-authored-by: Petr Viktorin <encukou@gmail.com>