Add Json.Decode.enum

[BrianHicks]

This PR implements Json.Decode.enum, which is enum : Dict String a -> Decoder a.

You use it like this:

funnyBool : Decoder Bool
funnyBool =
  enum <|
    Dict.fromList
      [ ("yes", True)
      , ("no", False)
      ]

Alternatives:

• use a List (String, a) instead, which makes construction a little easier
• just do this yourself with andThen. But, context in Slack, it's useful in a lot of places.

Context: https://elmlang.slack.com/archives/C13L7S5GR/p1502488068299237

Ellie with this working: https://ellie-app.com/47dBBznjnHWa1/1

[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.

[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.

[evancz]

Can you share a few examples of this with me in one of the slack channels? I think it's a fine time to add it to elm-lang/json, but I want to do a bit more thorough review first.

[evancz]

Can you share a few examples of this with me in one of the slack channels? I think it's a fine time to add it to elm-lang/json, but I want to do a bit more thorough review first.