[ConstraintSystem] Use sparse representation for constraints. (NFC)

Update ConstraintSystem to use a sparse representation for entries in a
row. Most rows only contain a small number of variables, so the sparse
representation can result in significant speedups.

For a large test case from D135915, it halves the time spent in
ConstraintElimination.

To ensure this returns the same results as the old implementation in all
cases, I built a large set of projects with an extra assertion that it
produces the same result as the old implementation.