Add cats.Parallel instance for Rerunnable

[felixbr]

This is an attempt to do a similar implementation as with Future to complete issue #90.

I want to stress that I've never used Rerunnable. My guess is the semantics are similar to cats.effect.IO (referentially transparent, lazy, no memoization), but you might want to check if my implementation is what you want from Rerunnable.

I've used Future.join as a way to run two Rerunnable types in parallel. I haven't seen a direct way without Future. Please check if this is ok.

As mentioned in the last review the Newtype1 helper is only needed for 2.10, so I tried to use a more direct encoding this time. Maybe it's possible to do it with even less indirection, but I kept the code close to Future and cats.effect.IO for the time being.

Cheers
~ Felix

[codecov-io]

Codecov Report

Merging #161 into master will decrease coverage by 0.79%.
The diff coverage is 88.88%.

@@            Coverage Diff            @@
##           master     #161     +/-   ##
=========================================
- Coverage   94.78%   93.98%   -0.8%     
=========================================
  Files           5        5             
  Lines         115      133     +18     
  Branches        3        3             
=========================================
+ Hits          109      125     +16     
- Misses          6        8      +2

Impacted Files	Coverage Δ
...il/src/main/scala/io/catbird/util/Rerunnable.scala	90.24% <88.88%> (-0.39%)	⬇️

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[felixbr]

codecov complains about rerunnableParEq which I only added for consistency with Future. It's not used currently, so I could also remove it...

[travisbrown]

Sorry for the long delay in review, but thanks for doing this!