Use weak references in register_* functions so that garbage collection still works

[Zac-HD]

See #3131; this is a nice performance optimization for anyone registering lots of transient Random instances.

I experimented with using the same technique under register_type_strategy() etc., but there are so many can't-be-weak caches, closures, and references involved in strategy internals that there's no practical way to make it happen without serious performance degradation.

The exception to that is unwrap_cache, where if the outer strategy is dropped we certainly don't need to remember what it wrapped; it's a small but cheap memory-use reduction.

[rsokl]

Nice! This looks good, and it a nice improvement for heavy RNG users. The use of count is elegant!