Extract duplicated UTF-8 codepoint-to-bytes encoding into a shared
function in src/string.c. Update all gems to use it:
- mruby-sprintf: %c specifier
- mruby-io: putc
- mruby-string-ext: Integer#chr
- mruby-pack: pack("U")
- mruby-compiler: Unicode escapes in parser
Also use existing mrb_utf8len() in io.c for character length detection.
Co-authored-by: Claude <noreply@anthropic.com>
Yukihiro "Matz" Matsumoto
7e28e68dca