update multi-party-eddsa repo

[vherbert]

multi-party-eddsa (v0.2.1) employs curv (v0.2.0-ed25519) with features = ["ec_ed25519"].

This version of curv requires a version of zeroize package which does not exist anymore.

The lib curv (v0.6.2) employ more recent version of zeroize but do not use features anymore.

all features were removed from curv in past versions. We used to choose EC point implementation by specifying a feature, now all curve implementation are available in curv::elliptic::curves::*, and all structures and algorithms are generic over choice of curve (ECPoint trait). I.e. curv API has been changed and some manual actions are required to move on next version.