[m-rev.] for post-commit review: improve {,fat_}sparse_bitset.m

[Zoltan Somogyi]

For post-commit review by anyone.

I tested the change to sparse_bitset.m, but ran into Mantis bug 510
when trying to test fat_sparse_bitset.m. However, since I made the
two modules use the same algorithms, I believe the risk of committing
the change to fat_sparse_bitset.m as well to be negligible.

Zoltan.
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