This module adds UTF-8 support to Lua. It's compatible with Lua's
own string module, and passes all string and pattern matching tests
in Lua's test suite.

It also adds some useful routines against UTF-8 features:

- a convenient interface to escape Unicode sequences in strings.
- string insert/remove, since UTF-8 substring extract may be expensive.
- calculate Unicode width, useful when implementing e.g. console emulator.
- a useful interface to translate Unicode offsets and byte offsets.