In saw-core backends, subNat now returns 0 instead of error on underflow.
In saw-core backends, subNat now returns 0 instead of error on underflow.

This fixes the error message reported in GaloisInc/saw-script#40, but
the example there still doesn't work because of strictness problems.

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