Two versions of proof::Content serialization.

[dpc]

There are two, slightly different ways to serialize proofs:

• the final one: this one gets signed and becomes a proof
• the editing one: when interactively editing the proof, it makes sense to show/hide some fields differently / have different defaults:
  • version should not be editable by the user, but be there in the final serialization
  • comment should always be there when editing, but hidden if empty in the empty version
• maybe others
• there's probably no point in showing the digest and revision when editing (are we sure?)

The solution here is probably to have two versions of each struct, with different serde annotations, and some methods to conveniently convert them back and forth.