Equality check on Json.Decode.oneOf always returns true

[shuhei]

Currently any pair of oneOf decoders are equal in the equality check that is used for diffing in virtual-dom. Because of this, changes in oneOf decoders are not reflected in event handlers of DOMs. (An example on Ellie)

Equality of two oneOf decoders are checked with _Json_listEquality(), which expects JavaScript arrays of decoders. However, a oneOf decoder has an Elm List of decoders instead of a JavaScript Array. Because of this, the len variable below becomes undefined given oneOf decoders, and the function always returns true for oneOf decoders.

https://github.com/elm-lang/core/blob/bc067c8e3db20235fc275b1d825bb76f365a1509/src/Elm/Kernel/Json.js#L378-L398

[process-bot]

Thanks for the issue! Make sure it satisfies this checklist. My human colleagues will appreciate it!

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