polynomial-3: use a typedef of Polynomial<3>.

Since template polynomials are now available, I finished moving `Polynomial3` to `Polynomial<3>` with a simple typedef. I obviously kept the previous `Polynomial3` tests to ensure retrocompatibility.

The only things that may delay this PR are hypothetical performance issues. With `Polynomial<3>`, Eigen is used all the way, and most loops are known at compile time, so things should be fine, but a stress test would still be interesting to compare both implementations.