Better parameter storage and validation

[adr1anh]

• Update README with better description of new keygen.Config types
• Move Lagrange utilities to pkg/math/polynomial and optimize for the case where all coefficients need to be computed.
• Define new keygen.Config type described by a protobuf. This allows for better serialization.
• Better validation of participants and threshold
• Validation of Paillier (p,q,N) is done with public methods and does not require the key objects to be created.
• Un-export methods from party.IDSlice and remove Lagrange.
• Un-export Pedersen parameter from Parameters struct, and perform validation on creation.