perf: simpler code in countN

gol/types.go

@@ -72,24 +72,10 @@ func (a Arena) countN(x, y int) int {
 	ysize := len(a)
 	xsize := len(a[0])
 
-	xb := x - 1
-	if xb < 0 {
-		xb = xsize - 1
-	}
-	xa := x + 1
-	if xa == xsize {
-		xa = 0
-	}
-
-	yb := y - 1
-	if yb < 0 {
-		yb = ysize - 1
-	}
-
-	ya := y + 1
-	if ya == ysize {
-		ya = 0
-	}
+	xb := (x - 1 + xsize) % xsize
+	xa := (x + 1 + xsize) % xsize
+	yb := (y - 1 + ysize) % ysize
+	ya := (y + 1 + ysize) % ysize
 
 	neigh := [8]pair{
 		{xb, yb},