Preserve "step intertia" when calculating a next or previous date

[alonsodomin]

Fixes #50

Step intertia refers to carrying over the direction of the step operation across the different fields in the Cron expression even when the carry over value from the previous field is equal to 0. Ignoring this inertia results in invalid result values for those fields constrained by non-continuous sub-expressions and other potential side effects.

This PR fixes this problem and adds a regression test to prevent future ocurrences.

[codecov-io]

Codecov Report

Merging #52 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
+ Coverage   94.37%   94.39%   +0.01%     
==========================================
  Files          58       58              
  Lines         818      820       +2     
  Branches       44       32      -12     
==========================================
+ Hits          772      774       +2     
  Misses         46       46

Impacted Files	Coverage Δ
...hared/src/main/scala/cron4s/datetime/Stepper.scala	100% <100%> (ø)	✅
...ain/scala/cron4s/testkit/laws/EnumeratedLaws.scala	100% <100%> (ø)	✅
...ared/src/main/scala/cron4s/syntax/enumerated.scala	100% <100%> (ø)	✅
.../src/main/scala/cron4s/datetime/DateTimeCron.scala	86.36% <100%> (+0.31%)	✅
...main/scala/cron4s/testkit/discipline/package.scala	100% <100%> (ø)	✅
...shared/src/main/scala/cron4s/base/Enumerated.scala	100% <100%> (ø)	✅

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