feat(check): Remove the alias dependence for variants

Before a variant could only be defined at the top level of a type definition since they needed to construct an instance of that type. This lifts that restriction and variant constructors now returns a row type of `forall a . | VARIANT_NAME ARGS*  | a`. Which ends up meaning that they construct an open variant row which contains at least the constructor in question, but could also contain more constructors.

check/src/typecheck.rs

@@ -1984,13 +1984,6 @@ impl<'a> Typecheck<'a> {
                 self.translate_ast_type(&type_cache, typ)
             });
 
-            if let Type::Variant(ref row) = **alias.unresolved_type().remove_forall() {
-                for field in row.row_iter() {
-                    let symbol = self.symbols.scoped_symbol(field.name.as_ref());
-                    self.original_symbols.insert(field.name.clone(), symbol);
-                }
-            }
-
             // alias.unresolved_type() is a dummy in this context
             self.named_variables.extend(
                 alias

check/tests/pass.rs

@@ -987,6 +987,6 @@ else
 
     assert_req!(
         result.map(|t| t.to_string()),
-        Ok("forall a . | test.A Int | test.B String | a")
+        Ok("forall a . | A Int\n| B String\n| a")
     );
 }