Implement the Borrow trait for KeyRef so key references can come from any borrowed form of the key
Implement the Borrow trait for KeyRef so key references can come from any borrowed form of the key

This commit implements the Borrow trait for KeyRef so that functions which require a reference
to the cache's key can use borrowed form of the key. For example, currently the get method requires
a reference to a `String` and will fail to compile if passed a `&str`.

jeromefroe/lru-rs