This also includes a new type for Value String.
The tests fail now, although it parses successfully. I'll use a pretty
printer in next commit so that it's easier to spot the differences.
Onces this is working I'll add the rest of the escaped characters.
- Add token combinator to simplify whitespace handling.
- Simplify whiteSpace parsers.
- Add `optempty` to handle pure mempty cases. `empty /= pure mempty`.
- Use `between` combinators for brackets, braces and parens.
This also includes small adjustments to the AST.
