Speed up `SparseBitMatrix`.

Speed up `SparseBitMatrix`.

Using a `BTreeMap` to represent rows in the bit matrix is really slow.
This patch changes things so that each row is represented by a
`BitVector`. This is a less sparse representation, but a much faster
one.

As a result, `SparseBitSet` and `SparseChunk` can be removed.

Other minor changes in this patch.

- It renames `BitVector::insert()` as `merge()`, which matches the
  terminology in the other classes in bitvec.rs.

- It removes `SparseBitMatrix::is_subset()`, which is unused.

- It reinstates `RegionValueElements::num_elements()`, which #52190 had
  removed.

- It removes a low-value `debug!` call in `SparseBitMatrix::add()`.