Simplify execution by removing nextBlock

Everything is managed by the stack, including what the execute() function does.

src/engine/runtime.js

@@ -122,6 +122,7 @@ Runtime.prototype.getOpcodeFunction = function (opcode) {
 Runtime.prototype._pushThread = function (id) {
     this.emit(Runtime.STACK_GLOW_ON, id);
     var thread = new Thread(id);
+    thread.pushStack(id);
     this.threads.push(thread);
 };