Rename Executor to Driver

[romac]

Closes: #29

[codecov]

Codecov Report

Merging #50 (ae2423c) into romac/rust-state-machine (45d4575) will not change coverage.
The diff coverage is 50.00%.

@@                    Coverage Diff                    @@
##           romac/rust-state-machine      #50   +/-   ##
=========================================================
  Coverage                     84.01%   84.01%           
=========================================================
  Files                            24       26    +2     
  Lines                          1226     1226           
=========================================================
  Hits                           1030     1030           
  Misses                          196      196           

Files	Coverage Δ
Code/driver/src/driver.rs	86.16% <ø> (ø)
Code/driver/src/message.rs	100.00% <100.00%> (ø)
Code/round/src/state_machine.rs	71.58% <ø> (ø)
Code/test/src/client.rs	100.00% <ø> (ø)
Code/driver/src/event.rs	0.00% <0.00%> (ø)

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