Revise handler stack implementation.

The old implementation:
- Wasn't actually checking whether handlers had been removed before
  calling them.
- Could end up calling the same handler twice (if a handler was removed
  further down the stack, and the stack elements moved due the resulting
  splice.

Solution:
- Mark elements as removed and check.  Set their ids to null.
- Don't splice stack.

Also, optimisation:
- Removing the element at the top of the stack is still O(1).
- In Modes, reverse handlers before removing (so, more likely to hit the
  optimisation above).

For the record, the stable stack length at the moment seems to be about
10-12 elements.

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