Translate escaped octal Strings back to their octal values.
For example, "\45" should go back to being the specific value (a %).
Note that this currently only supports the viable range of octal for Java; namely
1 to 377. This is both because parsing Java is the main use case and Integer.parseInt
throws an exception when values are larger than octal 377.
Translate a set of codepoints, represented by an int index into a CharSequence,
into another set of codepoints. The number of codepoints consumed must be returned,
and the only IOExceptions thrown must be from interacting with the Writer so that
the top level API may reliable ignore StringWriter IOExceptions.