[ZSAs] Rewriting the consensus rules (#83)

This PR adds an explicit upper bound of $$2^{64} - 1$$ to the total
supply of issued assets in the specification of ZIP 227.

It also does a major rewrite of the consensus rule changes, bringing
them more in line with the protocol specification, and using MUST
terminology where necessary.

Furthermore, these changes make explicit the order of processing fields
within a transaction, and the order of updates to the issuance state.

Author: vivek-arte