Un-overload Object fields and pre-parse damage dice (refactor step 3)

Object.Arm carried four meanings in C (o_arm/o_charges/o_goldval plus
ring bonuses); it is now four fields: ArmorClass, Charges, GoldValue,
and Bonus. Stats.Arm becomes Stats.ArmorClass. The Charges()/GoldVal()
accessor pair is gone.

Damage dice strings ("1x4/1x2") are parsed once into DiceSpec — at
table definition for the bestiary and weapon tables, at creation for
items — instead of re-parsed with atoi on every swing as fight.c
roll_em did. ParseDice preserves the C parse semantics exactly,
including the junk-tolerant "%%%x0" bestiary placeholder and the
"000x0" flytrap reset (regression-tested in dice_test.go); the
flytrap's growing grip becomes DiceSpec{{VfHit, 1}}.

Save format bumps to 5.4.4-go3 (field renames and retypes would
silently zero under gob's match-by-name decoding).

No behavior change; full suite green.

game/things.go

@@ -69,11 +69,11 @@ func (g *RogueGame) invName(obj *Object, drop bool) string {
 	case KindArmor:
 		sp := it.Armors[which].Name
 		if obj.Flags.Has(Known) {
-			fmt.Fprintf(&pb, "%s %s [", num(aClass[which]-obj.Arm, 0, Armor), sp)
+			fmt.Fprintf(&pb, "%s %s [", num(aClass[which]-obj.ArmorClass, 0, Armor), sp)
 			if !g.Options.Terse {
 				pb.WriteString("protection ")
 			}
-			fmt.Fprintf(&pb, "%d]", 10-obj.Arm)
+			fmt.Fprintf(&pb, "%d]", 10-obj.ArmorClass)
 		} else {
 			pb.WriteString(sp)
 		}
@@ -83,7 +83,7 @@ func (g *RogueGame) invName(obj *Object, drop bool) string {
 	case KindAmulet:
 		pb.WriteString("The Amulet of Yendor")
 	case KindGold:
-		fmt.Fprintf(&pb, "%d Gold pieces", obj.GoldVal())
+		fmt.Fprintf(&pb, "%d Gold pieces", obj.GoldValue)
 	}
 
 	out := pb.String()
@@ -168,7 +168,7 @@ func (g *RogueGame) dropCheck(obj *Object) bool {
 		p.CurRing[hand] = nil
 		switch obj.RingKind() {
 		case RingAddStrength:
-			g.chgStr(-obj.Arm)
+			g.chgStr(-obj.Bonus)
 		case RingSeeInvisible:
 			g.unsee(0)
 			g.Extinguish(DUnsee)
@@ -180,9 +180,9 @@ func (g *RogueGame) dropCheck(obj *Object) bool {
 // newThing returns a new random thing for the dungeon (things.c new_thing).
 func (g *RogueGame) newThing() *Object {
 	cur := newObject()
-	cur.Damage = "0x0"
-	cur.HurlDmg = "0x0"
-	cur.Arm = 11
+	cur.Damage = dice("0x0")
+	cur.HurlDmg = dice("0x0")
+	cur.ArmorClass = 11
 	cur.Count = 1
 
 	// Decide what kind of object it will be; if we haven't had food for a
@@ -219,20 +219,20 @@ func (g *RogueGame) newThing() *Object {
 	case 4:
 		cur.Kind = KindArmor
 		cur.Which = pickOne(g, g.Items.Armors[:])
-		cur.Arm = aClass[cur.Which]
+		cur.ArmorClass = aClass[cur.Which]
 		if r := g.rnd(100); r < 20 {
 			cur.Flags.Set(Cursed)
-			cur.Arm += g.rnd(3) + 1
+			cur.ArmorClass += g.rnd(3) + 1
 		} else if r < 28 {
-			cur.Arm -= g.rnd(3) + 1
+			cur.ArmorClass -= g.rnd(3) + 1
 		}
 	case 5:
 		cur.Kind = KindRing
 		cur.Which = pickOne(g, g.Items.Rings[:])
 		switch cur.RingKind() {
 		case RingAddStrength, RingProtection, RingDexterity, RingIncreaseDamage:
-			if cur.Arm = g.rnd(3); cur.Arm == 0 {
-				cur.Arm = -1
+			if cur.Bonus = g.rnd(3); cur.Bonus == 0 {
+				cur.Bonus = -1
 				cur.Flags.Set(Cursed)
 			}
 		case RingAggravateMonsters, RingTeleportation: