Refactor interpreters + Change Fetch.traverse

[peterneyens]

• Create a interpreter package and split the three parts of the interpreter parts into three separate files.
• Refactor processMany and processConcurrent in interpreter.CoreInterpreter
• Fix a bug in the handling of FetchMany.
• Customize the implementation of Fetch.traverse to reduce the stack needed to simplify/optimize the resulting Fetch.

With moving the interpreters and the reformating, it is a bit difficult to review ...

[codecov-io]

Codecov Report

Merging #123 into master will increase coverage by 0.72%.
The diff coverage is 89.59%.

@@           Coverage Diff            @@
##           master   #123      +/-   ##
========================================
+ Coverage   77.27%    78%   +0.72%     
========================================
  Files          11     14       +3     
  Lines         264    300      +36     
  Branches        2      2              
========================================
+ Hits          204    234      +30     
- Misses         60     66       +6

Impacted Files	Coverage Δ
shared/src/main/scala/implicits.scala	100% <ø> (ø)	⬆️
shared/src/main/scala/datasource.scala	66.66% <ø> (ø)	⬆️
shared/src/main/scala/env.scala	50% <ø> (ø)	⬆️
monix/shared/src/main/scala/monix.scala	71.42% <ø> (ø)	⬆️
shared/src/main/scala/freeinspect.scala	61.53% <0%> (-11.19%)	⬇️
...rc/main/scala/interpreters/MaxBatchSizePhase.scala	100% <100%> (ø)
shared/src/main/scala/interpreters.scala	100% <100%> (+6.06%)	⬆️
shared/src/main/scala/cache.scala	84.61% <100%> (-7.06%)	⬇️
shared/src/main/scala/fetch.scala	82.92% <77.77%> (+2.92%)	⬆️
...rc/main/scala/interpreters/ParallelJoinPhase.scala	89.18% <89.18%> (ø)
... and 5 more

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[juanpedromoreno]

LGTM, thanks!