Added StackSafeMonad mixin

[djspiewak]

I'm tired of defining the same tailRecM over and over again when the underlying monad is already known to be stack-safe. This mixin provides a more convenient way of achieving this goal. Obviously, the implementation is only valid if the underlying monad has a stack-safe flatMap, but if it has a stack-safe flatMap, then this is almost certainly going to be the implementation. May as well reduce some of the boilerplate in that case.

[kailuowang]

👍

[codecov-io]

Codecov Report

Merging #1736 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1736      +/-   ##
==========================================
- Coverage   94.03%   94.02%   -0.01%     
==========================================
  Files         252      253       +1     
  Lines        4189     4185       -4     
  Branches      154      163       +9     
==========================================
- Hits         3939     3935       -4     
  Misses        250      250

Impacted Files	Coverage Δ
core/src/main/scala/cats/instances/future.scala	75% <100%> (-1.48%)	⬇️
core/src/main/scala/cats/StackSafeMonad.scala	100% <100%> (ø)
core/src/main/scala/cats/Eval.scala	96.96% <100%> (-0.14%)	⬇️
free/src/main/scala/cats/free/Free.scala	85.71% <100%> (-0.83%)	⬇️

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[djspiewak]

Merging with two sign-offs.