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/save.go

@@ -13,8 +13,10 @@ import (
 
 // saveFormatVersion identifies the snapshot layout; it changes whenever a
 // field rename or retype would make gob silently mis-decode an older
-// save ("go2": Object.Type became Kind ObjectKind).
-const saveFormatVersion = Release + "-go2"
+// save ("go2": Object.Type became Kind ObjectKind; "go3": Object.Arm split
+// into ArmorClass/Charges/GoldValue/Bonus, damage strings became
+// DiceSpec).
+const saveFormatVersion = Release + "-go3"
 
 // destRef encodes a monster's chase target (state.c rs_write_thing's
 // (kind, index) pairs).