Keep polynomial constant parameters during DKG

[xoloki]

The group key created during dkg is the sum of the constant parameters of the party polynomials. This change adds the ability to keep these constant parameters when rerunning dkg, so the group key stays the same. Tests show that it works on both the low level Signer interface and the high level state machines.

This change also adds common::Polynomial, in an attempt to fix the disparity between how we handle private and public polynomials. Private polynomials, which use Scalar for both parameters and args, have been using polynomial::Polynomial from an external crate. But we weren't using any of the advanced features from that crate, just calling eval to generate private shares. Public polynomials couldn't use the crate, because they have Point parameters but Scalar args, which the crate did not support. Also, we couldn't support the keep_constant feature with the crate, since it did not allow specifying or editing any of the polynomial parameters.

This change is now using common::Polynomial for private polynomials, but Vec<Point> for public polynomials (with some help from Polynomial to convert between the two). This is similar to how the code worked before this change, so should require few downstream changes.

[djordon]

Largely, looks good. I gave it a quick look though but will give another one later. Figured I'd add my questions and comments here until then.