Get DensePolynomial compiling, and running faster.

Previously, there were some compilation errors with DensePolynomial.
Also, when benchmarking the dense case it was still 10-20x slower than
the Commons-Math polynomial.

This commit gets addition and multiplication running faster.
Evaluation and derivation are close (although slightly slower).
Unfortunately, there seem to be some bugs with quotMod. This commit
does not fix those, so some tests are still failing in the dense case.

Future work:

1. Fix quotMod
2. Get the tests running with both dense and sparse
3. Benchmark both dense and sparse cases
4. Verify invariants for dense polynomial