Implement Drain and IntoIterator

We should implement the `Drain` and `IntoIterator` traits on `HashMap`!

Part of what is tricky about `Drain` is the contract for what happens when the user "does something weird". For example, what happens if I write the following code:
```rust
std::mem::forget(map.drain());
```
Is `map` now empty or full? What happens if I read _one_ element and then drop the `Drain`? Take a look at [`hashbrown::RawDrain`](https://github.com/rust-lang/hashbrown/blob/6f5826ed934a5e6342681c27618bea71f9b0c800/src/raw/mod.rs#L1339) for some inspiration. I'm genuinely not sure what the best way to express this is a concurrent map.

@soruh began an implementation in #33, but it has since run out of time to work on it. It may still be useful to draw inspiration from though, and has a fair amount of good discussion around the challenges involved.

Start by reading the discussion and partial implementation in #33, then compare its constraints with hashbrown::RawDrain. Work out and document the intended behavior for forgotten drains, partial consumption, and dropping Drain; done means HashMap supports Drain and IntoIterator with those semantics established.

Written by the indexing model from the issue text.