This introduces a new SignatureField class, which wraps all

serialization/deserialization of project signatures, and updates the

Version model to use it. Callers will now see the new signature

objects, and won't need to worry about handling serialized data in any

form. This gives us flexibility later to change the serialization format

without breaking any callers.
All code loading in or saving signature data has been updated to work

with the new objects.

Unit tests passed.
Tested some basic evolution operations, which appeared to work. There's

still further development needed to get everything into place.