Fix elaboration of recursive data types.

[benl]

At the moment, data type definitions to be elaborated must be in-order and not recursive. For example:

data Foo a 
  = FooInt Int

data Bar a 
  = BarFoo Foo

The definition for Bar must come after def for Foo, otherwise Source.Desugar won't have the correct kind for it. Should probably tie this elaboration to kind inference.

[benl]

If we put them the other way around

data Bar a 
  = BarFoo Foo

data Foo a 
  = FooInt Int

Then we get an silent, malkinded type expression in the .di file.

data Bar %rDK3 a
        = BarFoo {
                Main.Foo %rDK3;   <--- bad kind
        };

data Foo %rDK4 a
        = FooInt {
                Base.Int32 %rDK4;
        };

Test is T44-DataTypeOrder