Update the saw-core translation of Cryptol selectors.
Update the saw-core translation of Cryptol selectors.

Use sites are no longer expected to push selectors down into
functions and sequences; that has been taken care of inside
the Cryptol typechecker now for some time.

Add a case for selectors applied to newtypes.  Newtype values
are currently translated exactly as their underlying records, so
we can emit essentially the same code, we just need to look up
the field index in the newtype definition.

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