Replace old abc::unicode_translate_error with abc::text_encode_error

The exception type abc::unicode_translate_error was deleted a while ago,
but this instance due to conditional compilation.
Add missing const qualifier to abc::to_str_backend char{8,16,32}_t [] specializations

The specializations for char{8,16,32}_t * are const, so this change is
consistent with the intent of making string literals directly convertible
without using one of the string classes.
Remove unnecessary namespace prefix