Speed up incVars, instantiateVar, and instantiateVarList.
Speed up incVars, instantiateVar, and instantiateVarList.

The generic internal function used to define all of them now has
a special case for closed terms, which map to themselves.

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