`entry2` documentation makes incorrect claim

[apasel422]

The following claim is incorrect:

Only copies the key if a new entry is inserted.

In particular, search_entry_hashed2 calls IntoCow::into_owned if the key is not found, even if VacantEntry::insert is not called. Ideally, VacantEntry would be changed to delay copying until insert is called.

[apasel422]

@gankro Any ideas on how we can do this most flexibly? I'm struggling to come up with a set of bounds that maintains the above while also permitting these possibilities:

let owned: String;
let borrowed: &str;
let owned_ref: &String;

let map: HashMap<String, i32>;
map.entry2(owned);
map.entry2(borrowed);
map.entry2(owned_ref);

let map: HashMap<&str, i32>;
map.entry2(borrowed);
map.entry2(owned_ref);

Edit: In particular, I think it's going to be an issue that String does not implement Borrow<&'a str> (which totally makes sense). We might need higher-ranked lifetimes for this?