Remove catalysts dependency

[travisbrown]

The catalysts dependency has proven to be a drag on every update (right now it seems to be blocking a release for 2.13.0-RC1) and at this point it's almost completely unused, so I'm proposing that we just scrap it.

There's some dead code in LawTests that uses catalysts's TypeTagM, but it is unused and hasn't been touched since 2016, so I just scrapped it. Otherwise the only thing catalysts is used for is to check whether the current build is Scala.js in order to set appropriate test sizes.

In my view this really should be done with separate code trees, but to keep this PR simple I just introduced a new (package private) cats.platform.Platform that duplicates the ~dozen lines of code Cats needs from catalysts.

MiMa says this change is fine, all tests pass as usual with no changes (apart from the Platform import), etc.

r? @SethTisue @kailuowang

[kailuowang]

Thanks so much for taking on this. I believe this is the right thing to do.

[SethTisue]

one less thing to worry about 👍

[travisbrown]

I ran validateJVM and prePR and thought that would be enough to confirm that the build works, but apparently it's not. Fixing the alleycats issue now (and trying not to get extremely frustrated that alleycats is used as a top-level package and shares no path with cats…).

[codecov-io]

Codecov Report

Merging #2791 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2791   +/-   ##
=======================================
  Coverage   94.28%   94.28%           
=======================================
  Files         367      367           
  Lines        6926     6926           
  Branches      297      304    +7     
=======================================
  Hits         6530     6530           
  Misses        396      396

Impacted Files	Coverage Δ
laws/src/main/scala/cats/laws/discipline/Eq.scala	33.33% <ø> (ø)	⬆️
...main/scala/cats/kernel/laws/PartialOrderLaws.scala	90.9% <ø> (ø)
...rc/main/scala/cats/kernel/laws/SemigroupLaws.scala	100% <ø> (ø)
.../scala/cats/kernel/laws/CommutativeGroupLaws.scala	100% <ø> (ø)
...ared/src/main/scala/cats/kernel/laws/package.scala	100% <ø> (ø)
...a/cats/kernel/laws/discipline/SemigroupTests.scala	100% <ø> (ø)
...kernel/laws/discipline/CommutativeGroupTests.scala	100% <ø> (ø)
...cala/cats/kernel/laws/discipline/MonoidTests.scala	100% <ø> (ø)
...cats/kernel/laws/discipline/SemilatticeTests.scala	100% <ø> (ø)
...cala/cats/kernel/laws/BoundedSemilatticeLaws.scala	100% <ø> (ø)
... and 24 more

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[tpolecat]

This is great, thanks @travisbrown 👍

[kailuowang]

I am on my phone, the code changes look good to me, but GitHub didn't really show the full path of the rename, so it appears that there is no actual path change. I am curious what they are.

[kailuowang]

replaced by #2792

[kailuowang]

@travisbrown I spoke too soon in #2792, there is a conflict and can't be resolved throught github ui. Would it be too much to ask you to resolve it?

[travisbrown]

@kailuowang Merged, thanks.