Make Equals compare by Ref (after comparing TypeRef)

This changes equal to compare by ref. Since computing the ref can be
expensive we first check that the type refs are equal.

Fixes #532

nomdl/codegen/set.tmpl

@@ -47,10 +47,7 @@ func (s {{.Name}}) InternalImplementation() {{$typesPackage}}Set {
 }
 
 func (s {{.Name}}) Equals(other {{$typesPackage}}Value) bool {
-	if other, ok := other.({{.Name}}); ok {
-		return s.Ref() == other.Ref()
-	}
-	return false
+	return other != nil && __typeRefFor{{.Name}}.Equals(other.TypeRef()) && s.Ref() == other.Ref()
 }
 
 func (s {{.Name}}) Ref() ref.Ref {