Change naming of hashes to digest

[ckartik]

The image above shows a pre-confirmation payload. You can observe, payloads are iteratively wrapped with integrity perserving hashes that form the digest input to signing operations.

Since hashes can have many use cases and does not provide a descriptive explanation for it's use in the context of the pre-confirmation structure, we should move towards using the word Digest in the context of the payload.

This will be increasingly important, as the commitment hash (to be named commitment digest) will also come to represent BidID in the context of this Update to ensure funds movement with a bid and future features sets centered around canceling and doing further operations on bids.

Using commitment Digest is a useful Bid ID, because:

1. It is unique to the expression of a Bid by a bidder.
2. It can be computed client-side by the bidder and any other party, that has access to the Bid Payload (which includes the bidder signature)