Cleanup escaped and characterClassEscape types

This fixes #42.

I am wondering if keeping `createCharacterClassEscape(value)` is worth it. In theory, it is possible to express `/\w/` as an AST of `/[A-Za-z0-9_]/`. However, I feel representing `/\w/` in this form in the AST is worth it.

In case anyone feels different, let me know.