Add eval overloads and loop ops

[eelstork]

• Eval(...) overloads are provided for certainties. Helps with producing nice logs when using certainties.
• Loop type now supports +/- operators.
• Improve test coverage

[codecov-commenter]

Codecov Report

Merging #81 (0aecfc9) into master (7177b8c) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #81   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           55        55           
  Lines          612       649   +37     
  Branches       107       107           
=========================================
+ Hits           612       649   +37     

Impacted Files	Coverage Δ
Src/Certainties/Action.cs	100.00% <100.00%> (ø)
Src/Certainties/Failure.cs	100.00% <100.00%> (ø)
Src/Certainties/Impending.cs	100.00% <100.00%> (ø)
Src/Certainties/Loop.cs	100.00% <100.00%> (ø)
Src/Certainties/Pending.cs	100.00% <100.00%> (ø)
Src/Stateful/Task.cs	100.00% <100.00%> (ø)
Src/Static/ActiveRaw.cs	100.00% <100.00%> (ø)
Src/Static/ActiveStatus.cs	100.00% <100.00%> (ø)
Src/Status.cs	100.00% <100.00%> (ø)

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