Rename item-subsystem methods (refactor step 5, items)

getItem→promptPackItem now returns (obj, ok) instead of a nil-signaling
pointer; invName→inventoryName; doPot→applyPotionFuse. C breadcrumbs
kept. Behavior unchanged; suite green.

game/misc.go

@@ -236,8 +236,8 @@ func (g *RogueGame) findObj(y, x int) *Object {
 
 // eat lets her try to eat something (misc.c eat).
 func (g *RogueGame) eat() {
-	obj := g.getItem("eat", KindFood)
-	if obj == nil {
+	obj, ok := g.promptPackItem("eat", KindFood)
+	if !ok {
 		return
 	}