Add ABCI crate

[thanethomson]

Closes #29

This PR contains experimental code that has slightly different aims to the other ABCI crates. I'll elaborate further as I've got more code.

• 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 #728 (9de5b8d) into master (68bb2e8) will increase coverage by 0.9%.
The diff coverage is 85.8%.

@@           Coverage Diff            @@
##           master    #728     +/-   ##
========================================
+ Coverage    42.7%   43.7%   +0.9%     
========================================
  Files         202     205      +3     
  Lines       12858   12889     +31     
  Branches     3223    3365    +142     
========================================
+ Hits         5496    5635    +139     
+ Misses       7014    6904    -110     
- Partials      348     350      +2     

Impacted Files	Coverage Δ
abci/src/result.rs	0.0% <0.0%> (ø)
proto/src/lib.rs	76.3% <ø> (ø)
abci/src/protocol/abci.rs	84.6% <84.6%> (ø)
abci/src/protocol/tsp.rs	88.8% <88.8%> (ø)
abci/src/lib.rs	100.0% <100.0%> (ø)
tendermint/src/public_key.rs	55.8% <0.0%> (-5.5%)	⬇️
light-client/src/components/verifier.rs	91.1% <0.0%> (-3.0%)	⬇️
light-node/src/rpc.rs	52.0% <0.0%> (-1.5%)	⬇️
light-client/tests/light_client.rs	86.0% <0.0%> (-1.3%)	⬇️
tendermint/src/account.rs	79.0% <0.0%> (-1.3%)	⬇️
... and 22 more

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[thanethomson]

I'm closing this PR in favour of another one I've been working on (for simplicity of review).