Improve `scAbstractExts` and `scGeneralizeExts`
Improve `scAbstractExts` and `scGeneralizeExts`

Previously, these functions were not working correctly when
the _types_ of some `ExtCns` values mention some of the
`ExtCns` values being abstracted or generalized.  Now we are
careful to both evaluate inside the types of `ExtCns` values
occuring inside the term, as well as making sure that the
types of `ExtCns` values in the list are processed properly
in the context of the outer values.

GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script