Implement print

[rbartlensky]

This PR adds an implementation of the print function, and extends the VM to allow closures to have a handler. A handler is a function defined in the vm, which is associated to a LuaClosure. If a closure is called and has a handler, the vm will call a rust function (which is the handler) instead of calling vm.eval.

The integration tests will be improved in my next PR where I will add the assert function.

[rbartlensky]

Addressed all comments. I've added some extra explanations for the handler. Ready for another review.

[rbartlensky]

Ok, so now I have a LuaClosure trait object, which has the same methods as before, but has an extra call method. This call method is different depending on the underlying object. A UserFunction will call vm.eval, and a BuiltinFunction will call (self.handler)(vm). One problem with this approach is that I can't have a Gc<LuaClosure>, only a Gc<Box<LuaClosure>> which means extra indirection. Also, when handling calls I have to do an explicit clone of the current closure, however, Gc::clone simply does a Cell::new, which AFAIK doesn't involve any heap allocation, so this operation is quite cheap. Why do I need to clone? This is because call takes a mut& Vm, thus vm.closure.call(vm) will trigger a borrow checker error.

[rbartlensky]

Ready for another review!

[ltratt]

Please squash.

[rbartlensky]

Done!