If we create a key but don't insert it (due to other Ractor winning the
race), then it would leak memory if we don't free it. This introduces a
new function to free that memory for this case.

We should never modify rb_concurrent_set_funcs during runtime, so we can
make it const.

There's nothing ractor related in them, and the classic terminology
for these sort of data structures is `concurrent-*`, e.g.
concurrent hash.