keen/string/unicode

Source
invalid-unicode-character record case
character nat32
show string(a invalid-unicode-character)
unicode-decode-error record case (has private fields)
show string(_ unicode-decode-error)
to char32(a char8)
to char8(a char32)
to nat32(a char32)
to char32(a nat32)
to string(a char32)
show string(a char32)
to char32[](a string)
to string(a char32 array-view)
to char8[](a char32)
next-char32 char32(iter char8 array-iterator)
prev-char32 char32(iter char8 array-iterator)
is-valid-unicode-character bool(a char32)