abci: Change hashes' type from bytes::Bytes to tendermint::Hash

[thanethomson]

Version(s) of tendermint-rs: master

Description

Related to #1090.

At the moment, several data structures in the tendermint crate's abci module have hash members of type bytes::Bytes (e.g. BeginBlock). I'd like to rather make these of type tendermint::Hash.

We can easily provide a conversion for non-empty hashes to bytes::Bytes.

cc @hdevalence

Definition of "done"

When all hash fields in structs in the tendermint::abci module are of type tendermint::Hash.

[mzabaluev]

Questions:

• The response structures Info and InitChain implement Default (Info also defaults the missing field values to the ones provided by the Default impl). If we change the app hash fields to AppHash, is it OK to have the default value of an empty byte array, like it already is de-facto?
• The hash field of Snapshot is documented as an arbitrary value compared by equality, so the semantics for neither tendermint::Hash nor tendermint::AppHash obviously apply. I think it's ok to leave it as Bytes.