Feature request: Retain Iterator

[Popog]

Have retain return a iterator over the extracted values, similar to Drain in the standard library, or my PreRFC.

It seems like a relatively simple change, so if you're interested I could probably get it done and shoot you a PR later this week.

[bluss]

Since this is in the interest of demoing it for libstd, it makes sense. It's not good if we have an iterator that doesn't adhere to the element order of the map, though.

[bluss]

Removing the breaking label because retain() should stay as it is (compatible with HashMap), but we can add new methods.

[cuviper]

The current equivalent in std would be HashMap::drain_filter, but this hasn't been stabilized yet.

[cuviper]

We'll let #242 track drain_filter.