I found that "x = y" invoking custom code brings serious consequences.

Jellyfish treats `x = y` as destruction of x, followed by a copy ctor.

For a brief moment of time, `x` is in an indeterminate state. This is not
a problem for memcpy, as no other part of the program can observe this brief
inconsistency. But if we allow custom operator=, then this incosistency may
become visible to the rest of the program if = does some funny stuff.

Consider

	struct Foo{
		field bar:Bar;
	}

	function f(x:ref Foo){
		x->bar = make_bar();
	}

In this case, the `Bar` destructor is invoked first. So `x->bar` becomes
uninitialized. Then it invokes a copy ctor, but if there are more references to
`x`, and this copy ctor uses one of them to access it, then they'll see `Foo`
in an incosistent state.