Implement the standard Charset encoders and decoders.

* US-ASCII
* ISO-8859-1
* UTF-8
* UTF-16 BE/LE/auto

This also fixes #1087: String.getBytes().
Eh!? Of course not! I'm delegating to the `s` interpolator, which will use `sc.parts` as needed. `parts` is never empty, btw, it always contains at least one string (when there is no `$`).