fix: canonicalize generic field objects for struct hashing

Use `types.Var.Origin()` when mapping and hashing struct fields so selector and
declaration sites share the same canonical field identity. This fixes build
failures for generic aliases and named types over those aliases.

Fixes #924

testdata/script/typeparams.txtar

@@ -16,6 +16,11 @@ func main() {
 
 	g2 := GenericGraph[*[]byte]{Content: new([]byte)}
 	g2.Edges = make([]GenericGraph[*[]byte], 1)
+
+	var ga genericAlias
+	ga.list = nil
+	var gan genericAliasNamed
+	gan.list = nil
 }
 
 func GenericFunc[GenericParamA, B any](x GenericParamA, y B) {}
@@ -66,3 +71,10 @@ func (w *AsyncResult[oldT, newT]) AsResult() Result[newT] {
 }
 
 func (w *AsyncResult[oldT, newT]) Redirect(struct {ret newT}) {}
+
+type genericAlias = generic[int]
+type generic[T any] struct {
+	list *T
+}
+
+type genericAliasNamed genericAlias