Rename Woodchipper to ConstantVisitor, and simplify the vendor/constants.js interface

I think "constant propagation" is the best term for what this module is doing, so I've altered the terminology to reflect that intent: http://en.wikipedia.org/wiki/Constant_folding#Constant_propagation

While I was at it, I simplified the `require("vendor/constants").propagate` interface to be synchronous instead of requiring a callback.