Avoid kind-projector syntax with variance annotations

[travisbrown]

Dotty currently reserves * for use as a type lambda placeholder for compatibility with kind-projector's syntax on Scala 2, via a currently-unimplemented -Ykind-projector compiler option. It's not clear whether -Ykind-projector should or will support kind-projector's λ[`-α` => Whatever[α]] syntax, or even -*. (I'm working on implementing -Ykind-projector at the moment, and personally I don't see compatibility with the goofier corners of kind-projector's syntax as a priority.)

Cats only uses kind-projector's variance annotations in this one file, and they're pretty easy to replace, and in my view the vanilla Scala syntax is clearer, anyway.

/cc @smarter

[travisbrown]

Also /cc @djspiewak.

[codecov-io]

Codecov Report

Merging #3207 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3207   +/-   ##
=======================================
  Coverage   93.05%   93.05%           
=======================================
  Files         376      376           
  Lines        7412     7412           
  Branches      192      192           
=======================================
  Hits         6897     6897           
  Misses        515      515

Flag	Coverage Δ
#scala_version_212	93.35% <100%> (-0.03%)	⬇️
#scala_version_213	92.82% <100%> (ø)	⬆️

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

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[joroKr21]

The reason why kind projector exists is for the cases where you can't use a type alias (in type signatures).

It's funny how once we have it we forget that in all other cases you can just use a type alias.

👍 looks clearer now.

[kailuowang]

seems an improvement to me.

[djspiewak]

This is definitely one of the corners of kind-projector that I don't care for at all. Also it has some weird caveats to it such that, in general, when you need variance or bounds or things, you usually end up being explicit with a type alias anyway. So I'm in favor of this change unambiguously.