Fixed all 2.13 warnings, reenabled fatalWarning on 2.13

[kailuowang]

fix #2893

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@207bb0d). Click here to learn what that means.
The diff coverage is 93.75%.

@@            Coverage Diff            @@
##             master    #2914   +/-   ##
=========================================
  Coverage          ?   94.24%           
=========================================
  Files             ?      364           
  Lines             ?     6951           
  Branches          ?      191           
=========================================
  Hits              ?     6551           
  Misses            ?      400           
  Partials          ?        0

Impacted Files	Coverage Δ
...rc/main/scala/cats/laws/discipline/Arbitrary.scala	99.15% <ø> (ø)
.../scala/cats/kernel/instances/StreamInstances.scala	100% <ø> (ø)
...n/scala/cats/kernel/instances/QueueInstances.scala	100% <ø> (ø)
...in/scala/cats/kernel/instances/ListInstances.scala	100% <ø> (ø)
core/src/main/scala/cats/Invariant.scala	90.47% <ø> (ø)
core/src/main/scala/cats/data/OneAnd.scala	95.94% <ø> (ø)
core/src/main/scala/cats/data/package.scala	88.88% <ø> (ø)
core/src/main/scala/cats/instances/parallel.scala	100% <ø> (ø)
.../scala/cats/kernel/instances/VectorInstances.scala	100% <ø> (ø)
...eycats-core/src/main/scala/alleycats/std/set.scala	51.16% <0%> (ø)
... and 16 more

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[kailuowang]

We might want more reviews. Merged for now to unblock folks at Scala matsuri contribution event.