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.