Use `HybridBitSet` for rows within `SparseBitMatrix`.

Use `HybridBitSet` for rows within `SparseBitMatrix`.

This requires adding a few extra methods to `HybridBitSet`. (These are
tested in a new unit test.)

This commit reduces the `max-rss` for `nll-check` builds of `html5ever`
by 46%, `ucd` by 45%, `clap-rs` by 23%, `inflate` by 14%. And the
results for the `unic-ucd-name` crate are even more impressive: a 21%
reduction in instructions, a 60% reduction in wall-time, a 96%
reduction in `max-rss`, and a 97% reduction in faults!

Fixes #52028.