Introducing a datatype for WorkLists that properly prioritizes equalities.
We were not prioritizing the interaction of equalities in the worklist, because pre-canonicalization solved the constraints one by one, in their arrival order. This patch fixes this, so it's a generally useful improvement, mainly for efficiency. It makes #4981 go away, although it's not a definite answer to the cause of the problem. See discussion on Trac.
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