Implement FromIterator for EntityHashMap

[alice-i-cecile]

          > This does change public API. Should a system be implemented to help migrate code?

Breaking changes are generally fine.

Perhaps an impl From<HashMap<K, V, S>> for EntityHashMap<K, V>

I think that we should probably just use the FromIterator trait. Useful idea though, might as well do that here.

Originally posted by @alice-i-cecile in #10415 (comment)

[nicopap]

EntityHashMap is a type alias for HashMap, so whatever HashMap implements, EntityHashMap does. What is needed here is change the API of methods accepting a EntityHashMap.