Add Client for getting a value and validate a proposal

[romac]

Closes: #27

At the moment, this solution prevents us from integrating with async code, so we'll have to revisit it if/when the need arises, or come up with something else entirely that is sync/async-agnostic. Maybe generators, or the maybe-async crate?

[codecov]

Codecov Report

Merging #43 (1c047e6) into romac/rust-state-machine (52d51a4) will increase coverage by 0.12%.
The diff coverage is 90.00%.

@@                     Coverage Diff                      @@
##           romac/rust-state-machine      #43      +/-   ##
============================================================
+ Coverage                     84.26%   84.39%   +0.12%     
============================================================
  Files                            23       24       +1     
  Lines                          1220     1236      +16     
============================================================
+ Hits                           1028     1043      +15     
- Misses                          192      193       +1     

Files	Coverage Δ
Code/test/src/client.rs	100.00% <100.00%> (ø)
Code/consensus/src/executor.rs	85.80% <81.82%> (-<0.01%)	⬇️

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