Encapsulate types that should be abstract for users of core

[jvoigtlaender]

For example, Form in Graphics.Collage is currently defined as

type alias Form =
    { theta : Float
    , scale : Float
    , x : Float
    , y : Float
    , alpha : Float
    , form : BasicForm
    }

and the type alias is exported from the Graphics.Collage module, so users get access to all the fields, but they are not documented anywhere (and might change in a future release). It should be better to hide them from the user of the library, then. So, replacing type alias Form = {...} by type Form = Form {...}.

The same applies to the Element and Position types in Graphics.Element.

[jvoigtlaender]

This is fixed in core-3.0.0.

[jvoigtlaender]

This is fixed in core-3.0.0.