More Cats instances

[sritchie]

This is a draft PR that adds instances for:

• Group[Real]
• Monoid[RandomVariable[A]]
• Monoid[Generator[A]]

Just experimenting with how much we can do here.

I'm running into this on RealGroup's tests:

[info] - Real.group.collect0 *** FAILED ***
[info]   ArithmeticException was thrown during property evaluation.
[info]     Message: Division undefined
[info]     Occurred when passed generated values (
[info]       arg0 = Constant(-1.1327590390904958E-124)
[info]     )
[info] - Real.group.combine all *** FAILED ***
[info]   ArithmeticException was thrown during property evaluation.
[info]     Message: Division undefined
[info]     Occurred when passed generated values (
[info]       arg0 = Vector()
[info]     )

[codecov-io]

Codecov Report

Merging #376 into develop will increase coverage by 0.18%. The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #376      +/-   ##
===========================================
+ Coverage    46.56%   46.75%   +0.18%     
===========================================
  Files           87       87              
  Lines         3118     3125       +7     
  Branches       136      138       +2     
===========================================
+ Hits          1452     1461       +9     
+ Misses        1666     1664       -2

Impacted Files	Coverage Δ
.../scala/com/stripe/rainier/scalacheck/package.scala	83.33% <ø> (+4.16%)	:arrow_up:
...e/src/main/scala/com/stripe/rainier/core/RVG.scala	0% <ø> (ø)	:arrow_up:
...rc/main/scala/com/stripe/rainier/core/Events.scala	0% <ø> (ø)	:arrow_up:
...c/main/scala/com/stripe/rainier/cats/package.scala	96.36% <100%> (+2.61%)	:arrow_up:
...ain/scala/com/stripe/rainier/compute/RealOps.scala	77.96% <0%> (ø)	:arrow_up:

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[sritchie]

Okay @avi-stripe, this should be my last one for a bit! I played around with adding a monad transformer for Generator, but I don't think it helps anything I'm doing. It's too fun to play around with this stuff.

[avi-stripe]

Awesome. Thank you!