Immediately stop listener iteration if the list is destroyed
Use rawget to ensure constructors are not invoked twice
Make object annihilation be optional

Annihilation (at least, that's what I call it) is a useful tool for
catching unexpected uses, but it can be a bit too strict. It seems more
useful to allow objects to opt-in to this protection, rather than
mandating it for everyone.
Pulled out drag threshold code into a separate callback
minor logging
minor whitespace
Add support for Rainback's drag and drop
Return from a Callbacks.Script callback

This is a noop for WoW, but it's useful for Rainback as it allows
handlers to ignore events
minor whitespace
Renamed FauxLuaEnvironment to NativeLuaEnvironment
Allow LuaEnvironments to ignore package.loaded
minor refactor + whitespace
Callbacks.DragThreshold fixes

I added a missing local to the conditional variable. I also propagate
arguments to both the conditional and the eventual threshold handler.