- Add missing variable parsing.
- Reuse `name` in value string.

This parses successfully the `kitchen-sink.graphql` sample from
`graphql-js`.

They are less efficient but they are giving me issues because they don't
fail. Once this is working I'll look into optimizing.

Also disable skipping comments until I figure out how to skip both
comments and space at the same time.

- 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.