Memoize result of binary logic operators on LargeBitsVal arguments.
Memoize result of binary logic operators on LargeBitsVal arguments.

Fixes #446.

This fix is very similar to the one for #514, which added memoization
for symbolic if-then-else on sequences. This is another instance where
a SeqMap is created where each lookup triggers two further lookups,
causing exponential runtime behavior in the worst case.

GaloisInc/cryptol,GaloisInc/cryptol,GaloisInc/cryptol