Cells can have their values set when ranging over them

plus more replacement of y, x loops with cell ranges, adding static
neighbor coords.

conway/conways-game-of-life/main.go

@@ -40,7 +40,12 @@ func main() {
 
 	checksums := make([]string, 4)
 	checksums[0], checksums[1], checksums[2], checksums[3] = "foo", "bar", "baz", "qwx"
-	pushChecksum(game.Checksum(), checksums)
+	ck, err := game.Checksum()
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "NAY NAY: %v\n", err)
+		os.Exit(3)
+	}
+	pushChecksum(ck, checksums)
 
 	ticks := time.Tick(time.Duration(*sleepMs) * time.Millisecond)
 	generations := 0
@@ -50,7 +55,11 @@ func main() {
 
 		game.EvaluateGeneration()
 
-		curChecksum := game.Checksum()
+		curChecksum, err := game.Checksum()
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "Failed to calculate current game checksum: %v\n", err)
+			os.Exit(4)
+		}
 
 		if checksums[0] == curChecksum || checksums[1] == curChecksum {
 			fmt.Println("Stasis!")