AK: Verify that functions aren't modified while they're being invoked

A Function object should not be set to a different functor while the
original functor is currently executing.

AK/Function.h

@@ -205,6 +205,7 @@ private:
     template<typename Callable>
     void init_with_callable(Callable&& callable)
     {
+        VERIFY(m_call_nesting_level == 0);
         using WrapperType = CallableWrapper<Callable>;
         if constexpr (sizeof(WrapperType) > inline_capacity) {
             *bit_cast<CallableWrapperBase**>(&m_storage) = new WrapperType(move(callable));