Bip174 extensions

This PR specifies 2 new types and clarifies multi-byte types as was discussed on the mailing list a few months ago.

A version type of 0xFB is added in the event that there is a breaking change to PSBT.

A proprietary type of 0xFC is added to allow for proprietary use types for use in private. This type is followed by an identifier string and then a vendor defined subtype.

Lastly, multi-byte types are clarified. Types are redefined to be minimally encoded compact size unsigned integers so they can be variable length now if we happen to run out of one byte types somehow.