V3 idea: Let Digest.hash use 'bytes' instead of 'string'

Right now the Digest message uses a string to store the checksum of the object. This is wasteful, because for the hashing algorithms that we use, the output is always a sequence of bytes. Using a bytes field cuts down the storage space by a half (for that field; not necessarily for the full object).

Start by locating the Digest message definition and reviewing how its hash field is represented and consumed. Check the API compatibility and migration implications of changing the field from string to bytes; done means the representation change is specified and its affected consumers and tests are identified.

Written by the indexing model from the issue text.