c255lbigintcmp without Math.round, Math.abs

c255lbigintcmp is commented "...tricky to avoid code branching", but it uses Math.round and Math.abs, which could very well have branches in their implementation. We can replace that with bit operations, which will be faster and less branchy.
Merge pull request #2 from ryancdotorg/patch-1

c255lbigintcmp without Math.round, Math.abs