two gob nits found with stricter 6g

(6g had a few pretty major bugs
where it was more lax than it should
have been when checking when things
satisfied interfaces; i fixed them and
this turned up.)

R=r
DELTA=2  (0 added, 0 deleted, 2 changed)
OCL=34243
CL=34248

src/pkg/gob/type.go

@@ -96,7 +96,7 @@ func (t *commonType) String() string {
 	return t.name
 }
 
-func (t *commonType) safeString(seen map[uint32] bool) string {
+func (t *commonType) safeString(seen map[typeId] bool) string {
 	return t.name
 }
 
@@ -146,7 +146,7 @@ func (a *arrayType) safeString(seen map[typeId] bool) string {
 }
 
 func (a *arrayType) String() string {
-	return a.safeString(make(map[uint32] bool))
+	return a.safeString(make(map[typeId] bool))
 }
 
 // Slice type