Move ObjectSpace#each_object to a JVM agent

[headius]

Currently we still have all the endpoints and checks in places for ObjectSpace.each_object, even if it is not enabled. This adds a small amount of overhead to most object allocations.

Since we require users to turn on the each_object functionality, we could at that point load an agent into the JVM that has access to debugging APIs, and use those APIs to provide the heap walking.

Work would need to be done to find the most efficient way to walk all heap objects.

My previous prototypes used the Java Debug Interface to loop over objects and update a separately-executing thread's active frame so it would see each object in turn. It worked, but was incredibly slow. An approach that instruments object construction and used the current weak map logic would be possible through an agent.

[headius]

This is more complicated than it seems due to the ability to turn on ObjectSpace.each_object at runtime via JRuby::Util.object_space=. I've got code in #8279 to officially deprecate that, but I'm unsure how frequently it might be used in the wild.