Validate record types to have no duplicate fields.

[mgold]

This is a redo of #1324: detecting duplicate fields in record types, whether in annotations or aliases. The detection happens during validation, so there's a very nice error and it's possible to get more than one at a time.

Example message:

-- DUPLICATE FIELD ---------------------------------------------------- test.elm

This record type has more than one field named `foo`.

9│ g : List { foo : String, foo : Int } -> Int
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^
There can only be one. Do some renaming to make sure the names are distinct!

Notice that it detected the duplicate even when nested in a list in a function, and also provides the exact location. This also works for aliases:

23│ type alias Foo = List { foo : String, foo : Int } -> Int
                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^

The implementation is adding another value constructor to Syntax.Error an detecting it in Validate. As we've seen, we need to recursively traverse raw types to ensure there are no invalid records hiding.

Adds two tests. Renames the value constructor for record values with duplicate fields for matching.

[evancz]

In my development branch, I am getting the following error messages for those cases:

-- NAME CLASH --------------------------------------------------------- temp.elm

This record has multiple `foo` fields.

4| g : List { foo : String, foo : Int } -> Int
              ^^^           ^^^
How can I know which one you want? Rename one of them!

-- NAME CLASH --------------------------------------------------------- temp.elm

This record has multiple `foo` fields.

9| type alias Foo = List { foo : String, foo : Int } -> Int
                           ^^^           ^^^
How can I know which one you want? Rename one of them!

The "duplicate name" errors should be much more precise in general though!