Could add support for additional ECC curves

[hko-s]

Specifically: p521, bp256, bp384

[dignifiedquire]

oh nice, yes we should add support for these, now that they are implemented in rustcrypto

[hko-s]

An update: p521 is now available for use with ecdsa via #304.

The current 0.6 versions of bp256 and bp384 seem tricky/not ready to use in rpgp. Seems to me these crates don't yet implement everything we need to use them in the same way that we use the nistp crates. For example, bp256::BrainpoolP256r1 doesn't implement elliptic_curve::CurveArithmetic.

I think the pragmatic way forward is to wait for more progress on the brainpool crates to land.

[hko-s]

nist p curves for ecdh are added in #351.