Hash should be base64 encoded in JSON

[matevz]

Export hash.Hash to JSON as string.

Currently, it's exported as array of bytes, for example
"state_root":[217,195,97,253,63,21,161,178,145,7,97,119,244,3,68,229,186,166,212,28,194,165,17,0,134,211,10,91,142,3,135,115]
It should be exported as a base64 encoded string, like "state_root":"jycFqjlMlRtaL9QhhK9gqVezw4Lp4Kro34MSH4YggS8="

[kostko]

This is probably the result of switching the codec library with the built-in one which doesn't use Base64 by default for types that implement BinaryMarshaler. We probably need to go around and add TextMarshaler implementations to our types.

[Yawning]

Increasing the priority of this because, the export format should be concrete sooner rather than later.

[matevz]

Do we also need to fix rust implementation anywhere?

[Yawning]

Do we also need to fix rust implementation anywhere?

If the rust code uses something that isn't CBOR, then yes, but as far as I am aware, this is not the case.

[matevz]

This is used by genesis-playback for generating genesis_dump.json: https://github.com/oasislabs/oasis-core/blob/master/runtime/src/common/crypto/hash.rs#L6

[matevz]

This is now also correctly implemented in rust as part of #2462