perf(rowids): count bitmap bits a word at a time

[pengw0048]

Problem

Bitmap::count_ones walks the bitmap one byte at a time. A row id sequence
asks for that count on the hot paths:

• U64Segment::len counts the whole bitmap of a RangeWithBitmap segment, and
  RowIdIndex::new calls len on every segment of every fragment.
• U64Segment::position counts the bits up to the wanted offset, once per
  lookup.

On a 15.43B row / 17,601 fragment dataset whose sequences hold 3.36 GB of
bitmaps, that byte-at-a-time count is the whole cost of opening the index.

Change

Count a u64 at a time, and keep the byte loop for the trailing bytes and for
the two partial bytes at the ends of a slice.

Measurement

Taking one row id from that dataset, release build, 96-core Linux x86-64. The
index here is the per-fragment index from #8624, which calls len per segment:

	before	after
first _take_rows, 1 row id	1.92 s	1.41 s
second call, same dataset object	1.10 s	0.61 s

cargo test -p lance-table --lib rowids: 98 passed.