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.h

@@ -58,6 +58,7 @@ err_t vm_execute_all(vm_t *);
 void vm_load_stack(vm_t *, byte *, size_t);
 void vm_load_program(vm_t *, inst_t *, size_t);
 void vm_load_registers(vm_t *, registers_t);
+void vm_stop(vm_t *);
 
 // Print routines
 #define VM_PRINT_PROGRAM_EXCERPT 5