added desugar: RegExpSyntax.syntax -> BasicRegExpSyntax.syntax

The regular expression library from SML/NJ is quite modular -- the
front-end can be used to parse extended regular expressions (including
sets "[a-z]", plus "a+", option "a?", etc) into an AST datatype which
can be "desugared" into basic regular expressions.

This isn't the most efficint approach; there are ways to implement
e.g. charsets while using much less memory, but it's theoretically
pleasant and easy.