Add memory tests

[lgarron]

We should check that ffi_free_memory_for_all_scramble_finders(…) actually works. Memory usage should go down to a few 'static allocations (e.g. parse_alg!(…) output) after calling it.

ffi_random_scramble_for_event(…) leaks its memory, we should also figure out the safest way to enable freeing that if the caller wants.