Allow using references when dealing with owned key types

E.g. if we have a `map: Map`, it would be nice to be able to access values using something like `self.map.get(&mut storage, "mykey")`, without the extra allocation/cloning.

@hashedone originally asked to investigate this.

Start by locating the Map type and its get method, then inspect how owned key types are handled and whether related lookup tests exist. The requested behavior is a borrowed-key lookup such as get(&mut storage, "mykey") without an extra allocation or clone; done means the API supports that example with appropriate coverage.

Written by the indexing model from the issue text.