…rule node children.

I need to profile this a bit more, but talos was pretty happy about this, and it
solves the known performance issues here such as the test-case from bug 1483963
for example. This also gets rid of a bunch of unsafe code which is nice.

This still keeps the same GC scheme, removing the key from the hashmap when
needed. I kept those as release assertions, but should probably be turned into
debug-only assertions.

Differential Revision: https://phabricator.services.mozilla.com/D6801