[Elm] Add support for array schemas

[lundal]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: 3.0.0 branch for changes related to OpenAPI spec 3.0. Default: master.
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Description of the PR

The following swagger.yaml currently produces models with invalid types and encoder and decoder functions. This change generates valid code by using arrayModelType, instantiationTypes, Decode.list and Encode.list.

swagger: '2.0'
info:
  title: API
  version: "1.0.0"
host: localhost
consumes:
  - application/json
produces:
  - application/json
paths:
  /test:
    get:
      summary: test
      responses:
        200:
          description: StringArray
          schema:
            $ref: '#/definitions/StringArray'
    put:
      summary: test
      responses:
        200:
          description: ObjectArray
          schema:
            $ref: '#/definitions/ObjectArray'
definitions:
  StringArray:
    type: array
    items:
      type: string
  ObjectArray:
    type: array
    items:
      $ref: '#/definitions/Object'
  Object:
    type: object
    properties:
      id:
        type: string

[wing328]

@lundal thanks for the PR.

cc @trenneman to review.

[eriktim]

Looking great, thanks @lundal !