Add hash_table::IterUnsafeMut, iter() method to various iterators

[clarfonthey]

Both of these would be useful for #666, and presumably other downstream hash table implementations.

First, the iter() methods make it possible to implement downstream Debug implementations more easily, by letting iterators like Drain be paused and viewed immutably.

Second, the new iter_unsafe_mut method and corresponding IterUnsafeMut exists to make implementing hash_map::IterMut a bit more reasonable without being as unsafe. The struct docs should hopefully make its existence clear, but I would definitely love some extra scrutiny on that.

[clarfonthey]

Since this is just an example, and not part of the actual implementation, I chose to use a clearer implementation instead of the one we actually use.

[Amanieu]

Rather than adding a whole new iterator kind, would it make more sense to direct users that need custom variance to the bucket iterator instead?

[clarfonthey]

That wouldn't really work because you would then need to mutably borrow the entire table inside the iterator, alongside the bucket iterator, to make it work, which would then give the same variance as IterMut. The only other option would be to make IterMut normally unsafe and variant, which feels like a worse option because I'd imagine a lot of code is just using IterMut and expecting the right thing to happen.

---

Also because you might be confusing the raw bucket API with the one for HashTable: it returns indices, not pointers, so, it's impossible to convert them back into references without the original table. And, mutably borrowing the table inside an iterator would make the whole thing invariant again, defeating the purpose of the variant version of the iterator.