Reimplement Nat-based shift/rotate primitives.
Reimplement Nat-based shift/rotate primitives.

Fixes GaloisInc/saw-script#150.
Fixes GaloisInc/saw-core#16.

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