Add memory tests

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.