Improve code quality on keygen output type

[marsella]

The output type for keygen is currently a tuple (Vec<KeySharePublic>, KeySharePrivate). However, we use this output in a few specific ways:

1. We use the whole thing as input to presign
2. We put together the public key shares to reconstruct the public key
3. Eventually (in #242), we'll use the public key shares as input to auxinfo

Since these uses are fairly constrained and they have to be done by the calling application, we should put them together in a nice, named, documented interface. This can live in the keygen module (probably keygen/participant.rs) but should be pub use'd in the main lib.rs.

• [x] Make the keygen output a named type instead of a tuple
• [x] Define a method to retrieve the set of public key shares as a list
• [x] Define a method to construct a k256::ecdsa::VerifyingKey from the set of public keyshares (like in the full protocol test at the bottom of protocol.rs).
• [x] If possible, reduce the visibility of the CurvePoint type to pub(crate) and remove the pub use for the type.
• [x] Add the new output type to the public API with pub use

[marsella]

Note on the last point about pub use -- we're talking in the example PR about changing the format of public types for the user. I'm not sure that we will actually have a decision by the time this PR gets addressed, but FYI.