Write tests

[bamless]

The language got big enough that tests are needed to ensure working state after changes. Writing some benchmarks would be nice too.

[jasmcaus]

Nice! Would you be interested in J*'s tests written using the tau testing framework? It's built with C in mind, it's fast and tiny (~1k lines). Moreover, it provides clean testing macros similar to Googletest (if you've used it in the past).