Specification of serialization format for IpaProof and MultiProof

[mratsim]

For interop, testing and differential fuzzing, it would be very helpful to have a specification of the IPAProof and MultiProof serialization format mentioned here:

https://github.com/crate-crypto/go-ipa/blob/408dbffb2041271c95979a3fb79d98b268bf2880/multiproof_test.go#L120-L121

Discussed with @agnxsh and @jsign following mratsim/constantine#350