remove the "open" concept from reflect and go with slices and arrays.

the two still share an interface and Kind; that's probably ok but might be worth revisiting.

R=rsc
DELTA=74  (1 added, 8 deleted, 65 changed)
OCL=23416
CL=23418

src/lib/reflect/tostring.go

@@ -81,7 +81,7 @@ func TypeToString(typ Type, expand bool) string {
 		return "*" + TypeToString(p.Sub(), false);
 	case ArrayKind:
 		a := typ.(ArrayType);
-		if a.Open() {
+		if a.IsSlice() {
 			str = "[]"
 		} else {
 			str = "[" + strconv.Itoa64(int64(a.Len())) +  "]"