fix FoxChickenCorn example
fix FoxChickenCorn example

The new type signature for popCount is simpler, and more importantly
passes the typechecker. The old one really _ought_ to typecheck, though,
so a shrunken version of it is now in the test suite for #126

GaloisInc/cryptol,GaloisInc/cryptol,GaloisInc/cryptol