Rollup merge of #143456 - joshtriplett:mbe-unused-rules-bitset, r=lqd

mbe: Change `unused_macro_rules` to a `DenseBitSet`

Now that it only contains indexes, and no other information, a bitset provides a more compact and simpler representation.

This builds on <rust-lang/rust#143416>. Only the last commit is new.

Author: jhpratt