This is in direct preparation for an automation mode of nix repl.

This parser can be reused for other purposes. It's inspired by
https://bitheap.org/cram/

Although eelco's impostor exists https://github.com/mobusoperandi/eelco,
it is not very nice to depend on out of tree testing frameworks with no
way to customize them.

This allows for automating using the repl without needing a PTY, with
very easy to write test files.

This is because they are unrepresentable in the source files with
commentary but not in the output, so we should just eat them in
normalization. It's ok.