Completely rewrite the code to convert JSVals to unions.

This code works correctly for the two unions currently in use, and raises
exceptions in codegen for most other cases. I will implement those cases when
the need arises.

src/components/script/script.rs

@@ -41,7 +41,6 @@ pub mod dom {
             pub mod PrototypeList;
             pub mod RegisterBindings;
             pub mod BindingDeclarations;
-            pub mod UnionConversions;
             pub mod UnionTypes;
         }
     }