Look into caching traits' "sealed" status to improve performance

[obi1kenobi]

Determining whether a trait is sealed requires determining whether any of its supertraits in the same crate are sealed. If checking traits' sealed status is slow, we may be able to cache those lookups.

Doing this while keeping the adapter and IndexedCrate types Send + Sync may be a bit of a challenge. Parallel hashmaps tend to introduce drop glue, which causes lifetime problems.