Reimplement varcreate and varstore

[Nickforall]

You should only be able to store when created.

var test = "kay"
test = "bye"

is completely fine as code

huh = "no"

isn't because huh is not initialized with var yet.

Currently the VM handles all assignments as a create.

[Nickforall]

I just realized this prevents me from doing the following:

var isConnected = false;
onEvent("connected", fn() {
    isConnected = true;
});

I'm gonna prioritize this.