Return Resident struct from iter and iter_mut

[DenizUgur]

This would help with #115 indirectly as I could insert to cache twice if ResidentState is Hot. I could then easily save and restore the cache. I would lose Ghost queue but at least residents would be restored.

This also helps if I need to now what hot items are in my cache if I need to perform some extra operation on them.

Edit: Upon further examining of the code I see the following logic, so if I were to iterate in reverse I would recreate the same cache anyway. My other point about knowing which item is hot or not is still a valid use case though.

quick-cache/src/shard.rs

Lines 1085 to 1095 in 8b0077f

	let enter_hot = self.weight_hot + weight <= self.weight_target_hot;
	// pre-evict instead of post-evict, this gives sightly more priority to the new item
	while self.weight_hot + self.weight_cold + weight > self.weight_capacity
	&& self.advance_cold(lcs)
	{}
	let (state, list_head) = if enter_hot {
	self.num_hot += 1;
	self.weight_hot += weight;
	(ResidentState::Hot, &mut self.hot_head)
	} else {