Handle Subroutines ABORT

[BasileiosKal]

2 sugestions for the Ciphersuite format:

A. Points serialization:

The point serialization we use is curve agnostic. No reason to be defined in the Ciphersuite. We could simplify things and define them as global parameters.

B. expand_len definition:

After #243 is merged, we can define expand_len in the ciphersuite and impose requirements for its value in a way that expand_message will never fail. We can then remove all the if uniform_bytes is INVALID, return INVALID steps (and potentially even make hash_to_scalar to never fail if we remove the check for 0s).

[BasileiosKal]

Discussed on WG on 6th of Mar. Will revisit for draft 03 after we simplify Sign

[BasileiosKal]

Discussed on WG call on 13th of Mar. The current point serialization cannot be curve agnostic. Will update the test vectors to use un-compressed format and re visit this after.

[BasileiosKal]

Discussed on WG call 20th of Mar. Will consider having PK as a point, to not restrict key representation.

[BasileiosKal]

Discussed on WG call on 27th of Mar. Consensus is to define ciphersuite specific encodings with the addition to define PKs as points in the operation's input

[BasileiosKal]

Discussed on the WG on the 22nd of May. Will address after PR #257 is merged