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/rings.go

@@ -49,7 +49,7 @@ func (g *RogueGame) ringOn() {
 	// Calculate the effect it has on the poor guy.
 	switch obj.RingKind() {
 	case RingAddStrength:
-		g.chgStr(obj.Arm)
+		g.chgStr(obj.Bonus)
 	case RingSeeInvisible:
 		g.invisOn()
 	case RingAggravateMonsters:
@@ -168,7 +168,7 @@ func ringNum(g *RogueGame, obj *Object) string {
 	}
 	switch obj.RingKind() {
 	case RingProtection, RingAddStrength, RingIncreaseDamage, RingDexterity:
-		return fmt.Sprintf(" [%s]", num(obj.Arm, 0, Ring))
+		return fmt.Sprintf(" [%s]", num(obj.Bonus, 0, Ring))
 	}
 	return ""
 }