feat: fast unused transitive imports

[kim-em]

This shares the work required to calculate the unused transitive imports of many modules at once. We need to use BitVec to represent subsets of modules (NameSet is too slow), and to avoid stack overflows we need to manage the stack manually.

[joneugster]

Didn't check every detail of the algorithm, but overall the PR seem good, and the command useful to have.

Thanks! Feel free to merge if you're done!