Decompose ObjectKind.String and doMotion (refactor step 7)

String's tail moves into stringRest; doMotion's erase step moves into
eraseFlight and the inverted loop drops a nesting level. Behavior
unchanged.

game/object.go

@@ -77,17 +77,9 @@ func (k ObjectKind) String() string {
 		return "suit of armor"
 	case KindRing:
 		return ringName
-	case KindWand:
-		return "wand or staff"
-	case KindAmulet:
-		return "amulet"
-	case KindGold:
-		return goldName
-	case KindRingOrStick:
-		return "ring, wand or staff"
+	default:
+		return k.stringRest()
 	}
-
-	return "bizarre thing"
 }
 
 // objectKindForGlyph is the reverse of Glyph: what category of item does a
@@ -123,6 +115,23 @@ func (k ObjectKind) MergesInPack() bool {
 	return k == KindPotion || k == KindScroll || k == KindFood
 }
 
+// stringRest names the remaining kinds, including the ring-or-stick
+// prompt pseudo-kind (the tail of the C type_name switch).
+func (k ObjectKind) stringRest() string {
+	switch k {
+	case KindWand:
+		return "wand or staff"
+	case KindAmulet:
+		return "amulet"
+	case KindGold:
+		return goldName
+	case KindRingOrStick:
+		return "ring, wand or staff"
+	}
+
+	return "bizarre thing"
+}
+
 // Object is the _o arm of the C THING union: anything that can lie on the
 // floor or ride in a pack.
 type Object struct {