Optimize BTreeSet intersections

[sistemd]

These intersections could be optimized if the blocks BTreeSet was represented with a bitset. Essentially, the blocks are always in the from_block..=to_block range, so each block can be represented as a bit, where the ith bit is on if the block from_block + i is present in the set. Then the intersection, cloned, and collect calls can be turned into a simple and operation on the bitsets.

This might not actually make a difference in practice, so we should measure carefully after doing this to decide if it's worth it.