Display `H256` instances in events as hex encoded string
Created by: ascjones
Following #547, instances of H256
in events are displayed as raw byte arrays. Like so (see Code::Stored::code_hash):
This PR introduces a customer decoder for H256
instances which displays it as a hex encoded string instead:
In doing this I have split the custom type transcoders into separate encoders and decoders so that a type can have only one or both.