Focus helpers for Array

Experimental package with a [focus] for [Array]. Note that this API is experimental and likely to go through many more iterations.

Feedback and contributions are very welcome.

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[][team] [team]: https://circuithub.com/about/team [focus]: http://package.elm-lang.org/packages/evancz/focus/latest [Array]: http://package.elm-lang.org/packages/elm-lang/core/latest/Array