Address "intended usage" descriptions in the protocol

[defuse]

Eli noted a lot of "intended usage" in our protocol spec. Basically, the protocol.pdf is written from an implementers perspective where the reader is assumed to be the full node and we discuss what the reader should / shouldn't do given what it sees from other parties in the protocol. In order to do a proper security review, the reader needs to make a mental leap along the lines of "I'm like this, and there are a lot of other people like this, but they might not behave exactly like this, and some might behave nothing like this."

We can fix this by separating the protocol into three parts:

1. Define what a full node's state is. We have the blockchain, the nullifier set, a list of unspent notes, etc.
2. Define the types of messages full nodes receive. Blocks, which contain transactions, which contain JoinSplits.
3. Define how those messages affect the full node's state. For example a new block will cause all of its transactions to get "executed" and upon execution a transaction can cause..., and so on.

This is sort of like what the Zerocash paper does, except (1) and (2) aren't separated in the Zerocash paper. An advantage is that there'll be a more 1:1 mapping between sections of the paper and parts of the code (e.g. in (1) we can list the files where those data structures are implemented). A disadvantage is that it would take me a lot longer to understand the protocol if I had to read it this way, which is why it took me so long to understand it from the Zerocash paper, but OTOH I would be able to implement it from the spec without even understanding it.

This is related to #32. I'm not sure if this would address Eli's comments, since transactions still have agency (they're seen as programs running on full node weird machines).

[defuse]

This would be a huge change to the spec and I'm not actually proposing we do it, just documenting the idea.

[daira]

I think there are irreconcilable conflicts between this and the abstract/concrete split that @elibensasson wanted, which is what I'm going with.