Add debug-only checks to expose all use-after-frees

[jonas-schievink]

With cfg(debug_assertions) or, since this is extremely expensive, only when a Cargo feature is enabled, we should perform a full collection every time a reference to a GC object is dropped.

This means that objects get freed as soon as the GC would be allowed to, which should expose all possible use-after-free bugs caused either by a bug in rcgc, missing roots, or incorrectly broken/moved object references.