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/potions.go

@@ -153,19 +153,19 @@ func (g *RogueGame) quaff() {
 		}
 	case PotionRestoreStrength:
 		if p.IsRing(Left, RingAddStrength) {
-			addStr(&p.Stats.Str, -p.CurRing[Left].Arm)
+			addStr(&p.Stats.Str, -p.CurRing[Left].Bonus)
 		}
 		if p.IsRing(Right, RingAddStrength) {
-			addStr(&p.Stats.Str, -p.CurRing[Right].Arm)
+			addStr(&p.Stats.Str, -p.CurRing[Right].Bonus)
 		}
 		if p.Stats.Str < p.MaxStats.Str {
 			p.Stats.Str = p.MaxStats.Str
 		}
 		if p.IsRing(Left, RingAddStrength) {
-			addStr(&p.Stats.Str, p.CurRing[Left].Arm)
+			addStr(&p.Stats.Str, p.CurRing[Left].Bonus)
 		}
 		if p.IsRing(Right, RingAddStrength) {
-			addStr(&p.Stats.Str, p.CurRing[Right].Arm)
+			addStr(&p.Stats.Str, p.CurRing[Right].Bonus)
 		}
 		g.msg("hey, this tastes great.  It make you feel warm all over")
 	case PotionBlindness:
@@ -212,7 +212,7 @@ func (g *RogueGame) doPot(kind PotionKind, knowit bool) {
 func (o *Object) isMagic() bool {
 	switch o.Kind {
 	case KindArmor:
-		return o.Flags.Has(Protected) || o.Arm != aClass[o.Which]
+		return o.Flags.Has(Protected) || o.ArmorClass != aClass[o.Which]
 	case KindWeapon:
 		return o.HPlus != 0 || o.DPlus != 0
 	case KindPotion, KindScroll, KindWand, KindRing, KindAmulet: