[Contracts] Experimental Slate v5

[yeastplume]

Slate V5 changes in order to provide experimental support to Early Payment Proofs for use with contract transaction workflows. This aims to provide the new slate fields in a V5 Slate as well as provide all needed v4 conversions and tests.

Specific slate changes are [WIP]:

PaymentInfoV5 Struct

• saddr, i.e. sender_address in main Slate becomes optional

• rsig is renamed to psig, corresponding to rename of receiver_signature to promise_signature in main Slate

• ts added (timestamp in main slate). Serialized as i64 representing epoch time in seconds

• memo added as optional MemoV5 Struct, which contains:

  • memo_type: u8
    • 0x00 = payment details directly embedded
    • 0x01 = Blake2b hash of an arbitrary invoice document
  • memo: [u8;32] the memo data itself

• [WIP] Outstanding task remains to implement additional tests that convert back and forth between slate versions.

[yeastplume]

Initial pass at defining all fields and serialization done, see edited comments above. Given that all versions of the slate previous to V4 were removed, should likely implement some tests to validate upgrading and downgrading of slates between V4 and V5

[yeastplume]

Going to merge shortly into contracts branch