When applying a defaulting substitution, keep going even if value found.
When applying a defaulting substitution, keep going even if value found.

Since our substitutions are idempotent, normally this would be just wasted
work, but when we are defaulting, we must make sure that we default the
varibales in the looked up thing also.

This fixes #534

GaloisInc/cryptol,GaloisInc/cryptol,GaloisInc/cryptol