Elm refactoring (#4891)

* [elm] Remove support for Elm 0.18

* [elm] Drop `Main.elm`

* [elm] Put all models in a single file

* [elm] Put generated code in `Api` package

* [elm] Introduce Request type for operations

* [elm] Generate list of enum variants

* [elm] Make default Time module

* [elm] Small improvements & fixes

* [elm] Improve support for allOf

* All of types are now nested;
* Adding a discriminator creates a custom type wrapping all variants
  and a 'catch-all' fallback variant.

* [elm] Add support for recursive types

* [elm] Add catch-all type for allOf

* [elm] Drop support for post-processing of files

* [elm] Only import required packages

* [elm] Handle reserved and unsafe words

* [elm] Minor improvements

* [elm] Add support for integer enums

* [elm] Remove additional slash in path

* [elm] Add link to docs

* [elm] Update CI test file & scripts

CI/circle_parallel.sh

@@ -21,9 +21,6 @@ elif [ "$NODE_INDEX" = "2" ]; then
     echo "Running node $NODE_INDEX to test ensure-up-to-date"
     java -version
 
-    # install elm-format
-    npm install -g elm-format
-
     # clear any changes to the samples
     git checkout -- .