proto: Rename DomainType trait to Protobuf

[thanethomson]

As discussed on our call yesterday, and towards fulfilling #654, in this PR I've:

1. Renamed the DomainType trait to Protobuf to clarify its purpose.
2. Kept it in the tendermint-proto crate, because it makes the most sense there.
3. Added a testable example of how to make use of the trait.

• Referenced an issue explaining the need for the change
• Updated all relevant documentation in docs
• Updated all code comments where relevant
• Wrote tests
• Updated CHANGELOG.md

[codecov-io]

Codecov Report

Merging #672 (e35f730) into master (fd3efe4) will not change coverage.
The diff coverage is 78.2%.

@@          Coverage Diff           @@
##           master    #672   +/-   ##
======================================
  Coverage    39.6%   39.6%           
======================================
  Files         183     182    -1     
  Lines       12873   12873           
  Branches     2944    2944           
======================================
  Hits         5104    5104           
  Misses       7522    7522           
  Partials      247     247           

Impacted Files	Coverage Δ
proto/src/error.rs	0.0% <ø> (ø)
tendermint/src/account.rs	80.2% <ø> (ø)
tendermint/src/block.rs	15.5% <ø> (ø)
tendermint/src/block/header.rs	89.0% <ø> (ø)
tendermint/src/block/height.rs	67.2% <ø> (ø)
tendermint/src/block/id.rs	73.6% <ø> (ø)
tendermint/src/block/parts.rs	68.5% <ø> (ø)
tendermint/src/block/size.rs	0.0% <ø> (ø)
tendermint/src/chain/id.rs	70.7% <ø> (ø)
tendermint/src/consensus/params.rs	0.0% <ø> (ø)
... and 18 more

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[greg-szabo]

Looks good. Maybe we should also rename the cases where there are multiple Error crates imported and the one from tendermint_proto::Error is aliased as DomainTypeError. A quick search showed me this in vote.rs, proposal.rs, sign_proposal.rs and sign_vote.rs.

[greg-szabo]

Good stuff.

I'm not sure if the word "equivalent" is the best to describe the similarities between the Protobuf::encode() and the Prost::Message::encode() functions. They have similar functionality that they implement differently. Maybe the word is part of some kind of documentation standard (you seem to have used the same template in multiple places) and I don't have a good enough vocabulary to give you a better alternative. Maybe "replacement"? Protobuf::encode() replaces the Prost::Message::encode() function with the added features of converting to a domain type. Or something.

If people understand "equivalent", then just ignore me.