Fix NestedIdOps.value leaking to types of shapes F[G[A]]

[zarthross]

Made 'value' private in NestedIdOps
This prevents 'value' from leaking into types with shape F[G[A]].

I encountered this issue while using Scalatests OptionValues on things with a G[A] shape, so I had a Option[G[A]] and tried to use .value. It caused an ambiguous implicit match with NestedIdOps.

I didn't add a test, because I didn't know where it should go. Would SyntaxSuite.testNested be an appropriate place?

[zarthross]

@kailuowang Should I have changed that to
private[syntax] val value: F[G[A]]
Instead?

If I'm reading this right, it avoids binary incompatibility?

[kailuowang]

Sounds a good plan.

[codecov-io]

Codecov Report

Merging #2423 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2423   +/-   ##
=======================================
  Coverage   95.27%   95.27%           
=======================================
  Files         351      351           
  Lines        6350     6350           
  Branches      274      274           
=======================================
  Hits         6050     6050           
  Misses        300      300

Impacted Files	Coverage Δ
core/src/main/scala/cats/syntax/nested.scala	100% <ø> (ø)	⬆️

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[zarthross]

@kailuowang I've fixed the binary compatibility issue, mind removing the label?

[ceedubs]

👍 LGTM.

Ideally we'd eventually want this to be changed back from package-private to private, wouldn't we? I'm wondering whether we should have a formal process for decluttering the noise that we add for binary compatibility (once we get to the point that we want to make a binary-incompatible release).