Structure query engine caches around DefId being a CrateNum & intra-crate DefIndex.

[eddyb]

Per-crate cache maps keyed on DefIndex may be more efficient and/or allow us to use Vec for queries keyed on DefId instead of HashMap.
We can use specialization (e.g. for splitting (DefId, T) into CrateNum and (DefIndex, T)), I think.

cc @nikomatsakis @michaelwoerister

[michaelwoerister]

Sounds like a good idea if the lookup array ends up being densely populated enough. In general, we should provide a way to implement customized in-memory caching for specific queries.