Use new elm-compiler parser

[avh4]

elm-compiler has a new parser, as of 2017-12. elm-format should switch to the new parser, as it is supposedly faster, and also gives much better error messages in most cases.

• [x] replace underlying primitives, and remove parsec https://github.com/avh4/elm-format/pull/754
• [ ] TBD: convert remaining parsers to use the new API directly (match elm-compiler's code style closely)
  • this will likely require https://github.com/avh4/elm-format/issues/787

[avh4]

Discussed this w/ Evan, and the plan here should be to copy over Primitvies.hs, and then implement all required Parsec primitives on top of the new Primitives, then drop parsec, and then the remainder of the parser can be incrementally rewritten to use the new primitives directly.