rewrite leak check to be based on universes

In the new leak check, instead of getting a list of placeholders to
track, we look for any placeholder that is part of a universe which
was created during the snapshot.

We are looking for the following error patterns:

* P1: P2, where P1 != P2
* P1: R, where R is in some universe that cannot name P1

This new leak check is more precise than before, in that it accepts
this patterns:

* R: P1, even if R cannot name P1, because R = 'static is a valid
sol'n
* R: P1, R: P2, as above

Note that this leak check, when running during subtyping, is less
efficient than before in some sense because it is going to check and
re-check all the universes created since the snapshot. We're going to
move when the leak check runs to try and correct that.

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