Optimize LookupSet before stabilizing

[austinabell]

With #654, it uses LookupMap with () as the value type, which can lead to some unnecessary reads rather than just checking if a key exists. There also might be some unnecessary CPU cycles to use borsh to deserialize nothing (not sure if compiler would optimize away)

We want to make sure we do this before stabilizing, in case we need/want to change the storage format.