Bug 1900050 - Optimize CycleCollectedRuntime's sweeping of nursery wrappers a little r=sfink,mccr8

This adds js::gc::InCollectedNurseryRegion which is fully inlinable and makes
JSObjectsTenured call the finalizers directly.

I don't think this makes a massive difference but it's still probably worth
doing.

Differential Revision: https://phabricator.services.mozilla.com/D212290