Switch from encoding/binary to struc

It's not perfect, but struc can deserialize the whole thing into one
struct while encoding/binary can't. It's nice to have that.

cmd/loader/main.go

@@ -139,16 +139,15 @@ func loadMapsFrom(mapsPath string) {
 	}
 
 	log.Printf("Maps in %s:", mapsPath)
-	for key, gameMap := range gameMaps {
-		rect := gameMap.Rect()
-		hdr := gameMap.Header
+	for key, gm := range gameMaps {
+		rect := gm.Rect()
 		fmt.Printf(
 			"    * `%s`: IsCampaignMap=%v W=%v:%v L=%v:%v SetName=%s\n",
 			key,
-			hdr.IsCampaignMap,
+			gm.IsCampaignMap,
 			rect.Min.X, rect.Max.X,
 			rect.Min.Y, rect.Max.Y,
-			string(hdr.SetName[:]),
+			string(gm.SetName),
 		)
 	}
 }