saw-core: Hash ExtCns values by name instead of VarIndex.

[brianhuffman]

This ensures that hashes of identical subterms will be stable across executions, and not depend on the particular VarIndexes that come from the hash-consing tables.

Fixes #2386.

[RyanGlScott]

I can see the appeal behind this, but on the other hand, I'm a bit uncomfortable about the Eq instance using ecVarIndex, but the Hashable instance using ecName. Surely the two ought to agree with each other?

[brianhuffman]

Using the VarIndex for comparisons is kind of the whole point of having a VarIndex at all. I ought to add some documentation explaining the global uniqueness invariant, so it's clear why comparing just the VarIndex suffices for the Eq instance. The rationale for the Hashable instance not depending on the VarIndex also belongs in a comment in the code.