Add a Reset method to the machine

creature.go

@@ -40,6 +40,13 @@ func (err Error) Error () (description string) {
 	return
 }
 
+// Reset resets the stack of the  machine. This should be called before Execute,
+// since Execute does not reset the stack.
+func (machine *Machine) Reset () {
+	machine.stack   = nil
+	machine.pointer = 0
+}
+
 // Execute starts execution at the address specified by offset. The machine will
 // run until a HALT instruction is encountered, or the end of program memory is
 // reached.