API migrations for Elm 0.19-rc

[avh4]

This is a tentative list from the limited info about Elm 0.19

• 🛑 remove elm install ordering workaround (blocked on Elm)
• add tests for non upgraded dependency
• remove elm-package.json when done
• add elm-lang/json if necessary
• install test dependencies (blocked on elm-test)
• add elm-lang/browser if necessary
• for packages, move elm files to src/ (or warn)
• move tests to tests/
• update readme
• for packages, warning if license is not in the whitelist
• if more than 5 exposed modules, give warning (see upgrade docs)

js stuff

• elm-package.json -> elm.json (See https://gist.github.com/eeue56/3aee215b53ecfd5b4de3cc13cb7b95ff via @eeue56 )
• package renames

elm-format stuff

• Tuple constructors (,,) are gone
• Basics.(!) is gone
• Basics.flip is gone
• Basics.curry is gone
• Basics.uncurry is gone
• String escape sequences "\u{xxxx}"
• large tuples are gone (should elm-format insert a warning comment?)
• .program -> Browser.*
• Json.parse returns Json.Error
• name shadowing is not allowed
• (maybe?) case on numbers becomes if/else?

[avh4]

Remaining items moved to #32