Example lexer and parser

[Kleidukos]

There should be an example lexer and parser included in the repo.

[sofia-m-a]

What would the ideal complexity for something like that be? I worry that too complex a language will distract from the error-reporting stuff, but too simple and there won't be many interesting kinds of errors to report.

We should try to think of some interesting errors to report, and make a language to lex/parse based on that :)

[Kleidukos]

@sofia-m-a A R⁵RS parser/lexer would be appropriate I think.

If you're feeling fancy, something with significant whitespace as well but that's a whole can of worms :|