Refactor mapping and lookup of federation objects

[gmac]

Per #108 (comment), it'd be nice to improve some logistics around how federation objects are mapped and then looked up. Some specific areas of improvement would be:

• Properly lookup namespace and interface locations rather handling them as error side-effects. This is done for interfaces in Refactor: interface selections have proper location #107.
• Pre-memoize abstract boundary implementations so that they don't have to be enumerated constantly.