Consistency with HashMap: missing `values` iterator.

[eddyb]

I'm trying to use ordermap to fix the non-determinism in rustbuild in a useful (order-preserving) way, and I had to replace .values() with .iter().map(|(_, r)| r) - it's close, but not a drop-in replacement.

[bluss]

Ok, values and values_mut should definitely be added so that it's a drop in. As noted somewhere, std HashMap's Entry has two type parameters and ours has three, which is a not yet fixed discrepancy