Add `into_keys` and `into_values` to associative maps

[orlp]

Associative maps (HashMap and BTreeMap) in Rust are currently missing two methods to get owning iterators to their keys or values.

We have iter to get key-value reference pairs, iter_mut to get key-value mutable reference pairs and into_iter to get owned key-value pairs.

I'd expect keys and values to offer a similar trio of methods (with an exception for keys_mut of course). values_mut is there, but the into variants are missing for seemingly no good reason.

As an example implementation (but perhaps we can do better):

pub fn into_keys(self) -> impl Iterator<Item = K> {
    self.into_iter().map(|(k, v)| k)
}

pub fn into_values(self) -> impl Iterator<Item = V> {
    self.into_iter().map(|(k, v)| v)
}

[dtolnay]

I would accept a PR to introduce unstable into_keys() and into_values().

[canova]

@dtolnay Hi, I’m interested in working on this but I have a question regarding the implementation. I guess we can implement this in two ways. First one is implementing like the example in the first comment, like returning Map<IntoIter<K, V>>. And the second option is to return a struct of its own (like a mixture of IntoIter and Keys/Values structs) instead of returning Map<IntoIter<T>>. We can name them IntoKeys and IntoValues and they can be responsible of implementing the Iterator trait.
I feel like performance wise they are same, since map and custom struct both will be evaluated lazily when we iterate over. But there might be a rule to always return a proper type in the rust std code that I don't know. So I think I’m okay implementing either way but wanted to ask just in case. What do you think?

[dtolnay]

For the public API I would prefer this implemented using dedicated IntoKeys and IntoValues iterator types. They can be implemented internally as wrappers around Map<IntoIter<K, V>> if that simplifies things.

[canova]

Thanks for the answer @dtolnay! Created a PR for this and added you as a reviewer. Would you mind taking a look at it when you have time?