HashMap: #or_default_entry and #or_insert_entry

[braddunbar]

I often find myself in a situation where I would like to insert a key into a map via #entry_ref and then use the key from the resulting entry.

// Get a byte slice from a buffer
let key = client.request.get(index);

// Insert the value (default)
let mut entry = match queues.entry_ref(&key) {
    EntryRef::Occupied(entry) => entry,
    EntryRef::Vacant(entry) => entry.insert_entry(Default::default()),
};

// Use the value
entry.get_mut().insert_back(client.id);

// Reuse the key instead of copying the bytes again
keys.insert(client.id, entry.key());

This is common enough that I'd love to have functions for it, similar to insert_entry.

// Get a byte slice from a buffer
let key = client.request.get(index);

// Insert the value (default)
let mut entry = queues.entry_ref(&key).or_default_entry();

// Use the value
entry.get_mut().insert_back(client.id);

// Reuse the key instead of copying the bytes again
keys.insert(client.id, entry.key());