remove eval

eval is unsafe, and the way it was being used in Script and Script interpreter
was not a good enough reason. This commit removes both uses of eval, then
replaces all uses of OP_XXX with Opcode.map.OP_XXX since there's no reason for
those constants to be global.
Merge pull request #321 from ryanxcharles/feature/remove-eval

Remove eval
remove unused chai dependency