Shared identity point trait

[daxpedda]

I found myself in a pickle when trying to generalize over both ProjectivePoint and AffinePoint.
Currently Group and PrimeCurveAffine have identity() and is_identity(), but there is no trait that can be implemented on both point types that provides access to this functionality.

So I would like to propose adding a new trait Identity and refactor identity() and is_identity() into it.
Happy to make the PR upon approval.

[tarcieri]

FWIW we use Default for this in all of the RustCrypto/elliptic-curve traits.

That said, it might be nice to use an associated constant like Identity::IDENTITY to make it const-friendly.