Explicit NEP on Tx Witness

[Igor Machado Coelho]

We should be able to mark witness VerificationScript by some "standard script" identifier.

Examples:
PUSH signature / Check signature - Standard Verification 1
PUSH signatures / Check multisig - Standard Verification 2
etc

Some time ago (months), we discussed on a similar situation, with @shargon, and he wisely commented that it wouldn't be interesting to have a NEP to inform things that could not be true... I mean, tx submitter may "lie" about which verification format is. In this case, I propose that nodes verify the format, and reject tx if a specific format is declared, but not followed.

Example:
declare multisig, but perform single sig

Positive points: this allows us to do some optimizations, specially for blockchain explorers, as they will better know witnesses intentions, and clearly indicate that (single sig / multi sig / etc).
This is important for this NEP: #102

Negative points: this is an extra byte (if we limit to 255) on all cosigners, unless we manage to keep it optional somehow.

[Roman Khimov]

Seems like everyone is just parsing scripts to detect this and (three years down the road!) we still have just two main possibilities, probably it's OK as it is.