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/move.go

@@ -241,7 +241,7 @@ func (g *RogueGame) beTrapped(tc Coord) TrapKind {
 		p.Flags.Clear(Awake)
 		g.msg("a strange white mist envelops you and you fall asleep")
 	case TrapArrow:
-		if g.swing(p.Stats.Lvl-1, p.Stats.Arm, 1) {
+		if g.swing(p.Stats.Lvl-1, p.Stats.ArmorClass, 1) {
 			p.Stats.HP -= g.roll(1, 6)
 			if p.Stats.HP <= 0 {
 				g.msg("an arrow killed you")
@@ -263,7 +263,7 @@ func (g *RogueGame) beTrapped(tc Coord) TrapKind {
 		g.teleport()
 		g.mvaddch(tc.Y, tc.X, Trap)
 	case TrapDart:
-		if !g.swing(p.Stats.Lvl+1, p.Stats.Arm, 1) {
+		if !g.swing(p.Stats.Lvl+1, p.Stats.ArmorClass, 1) {
 			g.msg("a small dart whizzes by your ear and vanishes")
 		} else {
 			p.Stats.HP -= g.roll(1, 4)
@@ -322,7 +322,7 @@ func (g *RogueGame) rndmove(who *Creature) Coord {
 // aren't wearing a magic ring (move.c rust_armor).
 func (g *RogueGame) rustArmor(arm *Object) {
 	if arm == nil || arm.Kind != KindArmor || arm.ArmorKind() == ArmorLeather ||
-		arm.Arm >= 9 {
+		arm.ArmorClass >= 9 {
 		return
 	}
 
@@ -331,7 +331,7 @@ func (g *RogueGame) rustArmor(arm *Object) {
 			g.msg("the rust vanishes instantly")
 		}
 	} else {
-		arm.Arm++
+		arm.ArmorClass++
 		if !g.Options.Terse {
 			g.msg("your armor appears to be weaker now. Oh my!")
 		} else {