Fix memory leaks

[kivikakk]

Fixes #16.

/cc @philipturnbull @mislav — I'm not an expert on Ruby extensions. In b456869 I've stored the state in a struct tracked by the GC and just left it to the GC to cleanup. In c8307a6 I use rb_protect instead and clean up. Either way preferred? The latter seems cleaner.

[kivikakk]

Confirmed by manual inspection this doesn't leak compared to master. (Ran the two test cases in a loop millions of times and watched master consume 100s of MB of RAM, whereas this one stays steady.)

[philipturnbull]

I'm not a Ruby extension expert either but I prefer the second approach fwiw. I used the same approach to fix a leak with VersionSorter.compare "\0" "\0"

That's all the leaks in #16 plugged ✨

[mislav]

I'm not a C expert at all, but I have confidence in your methodology. Merge away!