Present abc::*str elements as char32_t code points
Add char_t overloads to abc::*str methods taking a single char32_t
Add char_t overloads to abc::text::_codepoint_proxy methods taking char32_t
Remove unnecessary abc::text::codepoint() casts
Use abc::text::_codepoint_proxy consistently through all of abc::text::*codepoint_iterator*