tweak the Events API

[avi-stripe]

These are changes that were suggested by working on the Ch 5.1 notebook. Specifically:

• I've brought back the convenience methods that separate out a Seq[X] and Seq[Y]; these turn out to be a lot more readable IMO.
• Events.simulate has been moved to an object method that works with any X=>Generator[T], not just X=>Distribution[Y]. This is useful when you want to visualize counterfactual or predictive posteriors on functions that don't match the data's (X,Y) format, ie when you want (X,mu) in a regression.
• observe now just always returns the bare X => Distribution[Y] function rather than a case class wrapper

[codecov-io]

Codecov Report

Merging #379 into develop will decrease coverage by 0.01%. The diff coverage is 36.36%.

@@             Coverage Diff             @@
##           develop     #379      +/-   ##
===========================================
- Coverage    45.97%   45.95%   -0.02%     
===========================================
  Files           86       86              
  Lines         3080     3081       +1     
  Branches       134      129       -5     
===========================================
  Hits          1416     1416              
- Misses        1664     1665       +1

Impacted Files	Coverage Δ
...rc/main/scala/com/stripe/rainier/core/Events.scala	0% <0%> (ø)	:arrow_up:
...n/scala/com/stripe/rainier/core/Distribution.scala	83.33% <100%> (+8.33%)	:arrow_up:
.../main/scala/com/stripe/rainier/core/Discrete.scala	83.78% <100%> (-0.22%)	:arrow_down:
...ain/scala/com/stripe/rainier/core/Continuous.scala	80.95% <100%> (-0.23%)	:arrow_down:

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