HashMap::extend_with() to handle collisions

[nagisa]

HashMap provides a extend method, but that does not allow for handling the cases where key exists both in the hashmap and the iterator.

[apasel422]

I'd actually like to see an RFC for handling duplicate keys in general. The set types had rust-lang/rfcs#1194, but the map stuff was punted.

[dtolnay]

I believe this is going to require an RFC. Some things to cover:

• Concrete use cases that involve extending a HashMap with potentially duplicated keys, needing to handle those duplicates somehow.
• Proposed API for extend_with.
• What are the current workarounds without extend_with, and how extend_with improves upon them.
• How this could be generalized to other map types, or even other collection types that are not maps.