Expose Short Id on SessionContext

[benalleng]

My only other peice of feedback would be to expose the short id method on session context instead of any particular receiver type state. This out of the scope of this PR but worth noting.

Originally posted by @0xBEEFCAF3 in #642

[arminsabouri]

IIRC the original problem was this. Where the NS1R receiver needed to check against duplicate sender sessions. The NS1R reciever reads a v2::uncheckedProposal however only the previous type state had a id() method.

[benalleng]

Do you think its ok to keep SessionContext as just pub(crate) for this, any reason to extend this to be completely public?

[nothingmuch]

Do you think its ok to keep SessionContext as just pub(crate) for this, any reason to extend this to be completely public?

it should not be public IMO, only pub(crate)