Use different types for the executors inputs and outputs

[romac]

This allows to cleanly differentiate output messages which need to be acted on by a runtime (eg. an async runtime which would schedule a timeout) and input messages which are solely processed by the executor.

[codecov]

Codecov Report

Merging #17 (17ae258) into romac/rust-state-machine (4968797) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@                     Coverage Diff                      @@
##           romac/rust-state-machine      #17      +/-   ##
============================================================
- Coverage                     85.41%   85.35%   -0.06%     
============================================================
  Files                            17       17              
  Lines                           891      894       +3     
============================================================
+ Hits                            761      763       +2     
- Misses                          130      131       +1     

Files	Coverage Δ
Code/consensus/src/executor.rs	88.15% <100.00%> (-0.49%)	⬇️
Code/round/src/message.rs	66.67% <ø> (ø)

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