Encode digests in APIs and CLI using the same formatting (hexadecimal preferred)

[jllucas]

Digests are of "hashing.Digest" type (byte slice), which get applied a base64 encoding in a json serialization.
The goal is to avoid the base64 encoding.

[gdiazlo]

I think we need to implement marshalers and unmarshalers to our data types, so they get encoded as we want, instead of using the default rules of the json package.