Add `GCursor` and `KLens` types for traversing data with history#15

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mrdziuban wants to merge 3 commits intomasterfrom

Commits on Sep 25, 2020

Add `GCursor` and `KLens` types for traversing data with history; upgrade cats and scala-js.
mrdziuban
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Commits on Sep 28, 2020

Use `Self` type alias, use `moveList` for `downArray` too.
mrdziuban
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`downArray` => `downList`, add `Functor` and `Comonad`, bump versions.
mrdziuban
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