Implement May 3 updates to diff format in prop140
There are 4 changes to make here:
- The first digest in our diff code must correspond to the digest-as-signed of the consensus document.
- We must accept ,$d as an ed command.
- We must have the first command in all our generated diffs be a command that deletes the signatures.
- We must change our consdiffmgr index code to index diffs by the digest-as-signed of the original document.
Thanks to Taylor Yu and Alexander Færøy for helping me figure out the right solution here.