Add Map::get_key_value method.

[timothee-haudebourg]

I have added the get_key_value method to the Map type. This is useful to get a reference to the matched key that lives as long as the map.

[timothee-haudebourg]

Checks are not done yet, but I suspect it will fail with Rust below 1.40 where the get_key_value has been introduced in the standard library... Is backward compatibility below 1.40 a hard requirement?

[timothee-haudebourg]

I see you already use a configuration predicate called no_btreemap_get_key_value to use the get_key_value in Map::remove_entry. That would allow me to implement the get_key_value method only when it is available in the inner implementation, but I don't know anything about no_btreemap_get_key_value. Where is it defined?

[timothee-haudebourg]

So it works using no_btreemap_get_key_value, even though I don't know where this predicate is defined.

[dtolnay]

Thanks!