Added a routine to cleanup resources allocated to the VM

This means the stack should be heap allocated, which makes sense as
beyond 1KB one should really be using the heap rather than the stack.

vm/runtime.c

@@ -325,6 +325,13 @@ void vm_load_registers(vm_t *vm, registers_t registers)
   vm->registers = registers;
 }
 
+void vm_stop(vm_t *vm)
+{
+  free(vm->registers.data);
+  free(vm->program.instructions);
+  free(vm->stack.data);
+}
+
 void vm_print_registers(vm_t *vm, FILE *fp)
 {
   registers_t reg = vm->registers;