Fix equality check of oneOf decoder

[shuhei]

Fixes #904

• Renaming the existing _Json_listEquality as _Json_arrayEquality because it actually works on JavaScript arrays instead of Lists
• Adding a new _Json_listEquality function that works on Elm Lists

[process-bot]

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

Here is what to expect next, and if anyone wants to comment, keep these things in mind.

[rupertlssmith]

This is a good patch and fixes a subtle bug that people could spend hours figuring out.

This patch is old enough that in the mean time JSON stuff got moved to a separate package from core. Would you be able to open a new PR against elm/json for this?

elm/json#15

[shuhei]

Hi, thanks for looking into this old PR! I'm on vacation right now and also have been out of touch with Elm for a while. If you need this fix soon, please feel free to create a PR to the elm/json repo.

[rupertlssmith]

If you are out of touch with Elm, then I will create the PR in elm/json - your fix applies just the same in the new package.