Also implements ++ string concatenation operator. Resolves #7, #18.
In addition, for the sake of Haskell code generation, this PR adds static typing with Statix. Resolves #5. Co-authored-by: Glen Whitney <glen@studioinfinity.org> Reviewed-on: glen/fostr#19 Co-Authored-By: Glen Whitney <glen@nobody@nowhere.net> Co-Committed-By: Glen Whitney <glen@nobody@nowhere.net>