Removed deprecation of >> and changed its param to be a by-name

[mpilquist]

In FS2, we rely heavily on recursive functions that return values of type Pull[F,O,R]. These functions typically take a form similar to:

def foo: Pull[F,O,R] =
  if (guard) Pull.done else doSomeWork *> foo

Recently, @pchlupacek noticed that FS2 defines *> with a by-name param whereas cats defines *> with a strict param. Such recursive programs are broken if we use a strict param. Hence, we are going to change the FS2 definition of *> to be strict and reintroduce >> with a by-name param and then use >> in all recursive pulls.

Ideally, cats would have the same definition for >>, so in this PR I removed the deprecation of >> and defined it with a by-name param, making it a true alias for flatMap(_ => fb). Note that we already have followedByEval and forEffectEval, though the syntax overhead of using those is way too high IMO.

[LukaJCB]

I'm generally +1 on this, but should we maybe move it to Apply?

[mpilquist]

I thought about that but settled on leaving >> on FlatMap since the guarantee is that fb won't be computed until fa produces an A (precisely what flatMap describes).

[LukaJCB]

I see, yeah I think that makes sense! :)

[LukaJCB]

I think you'll need to add a mima exception

[mpilquist]

Yeah, just pushed a mima fix.

[codecov-io]

Codecov Report

Merging #2043 into master will increase coverage by 0.01%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #2043      +/-   ##
==========================================
+ Coverage   95.05%   95.07%   +0.01%     
==========================================
  Files         311      311              
  Lines        5255     5255              
  Branches      114      124      +10     
==========================================
+ Hits         4995     4996       +1     
+ Misses        260      259       -1

Impacted Files	Coverage Δ
core/src/main/scala/cats/FlatMap.scala	100% <ø> (ø)	⬆️
core/src/main/scala/cats/syntax/flatMap.scala	80% <50%> (+13.33%)	⬆️

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[johnynek]

This looks good to me.