Ignore phantom types when doing migration code gen

[MartinSStewart]

For types such as type Id a = Id String previously the following would get generated

migrate_Migrate_All_New_Id : (a_old -> a_new) -> Migrate_All.Old.Id a_old -> Migrate_All.New.Id a_new
migrate_Migrate_All_New_Id migrate_a old =
    case old of
        Migrate_All.Old.Id p0 ->
            Migrate_All.New.Id p0

With this PR, it will instead generate

migrate_Migrate_All_New_Id : Migrate_All.Old.Id a_old -> Migrate_All.New.Id a_new
migrate_Migrate_All_New_Id old =
    case old of
        Migrate_All.Old.Id p0 ->
            Migrate_All.New.Id p0

Note that currently this does not handle aliases. So something like

type alias Point2d units coordinates = Point2dHelper units coordinates

type Point2dHelper units coordinates = Point2dHelper { x : Float, y : Float } 

will still generate migrations for Point2d's units and coordinates even though they are phantom types.

[supermario]

Nice 👍