Replacing make with a direct buffer is faster.

This was a bit surprising, but seriously...

Before:
BenchmarkCommas  1000000  2974 ns/op

After:
BenchmarkCommas  1000000  2446 ns/op
diff --git a/comma.go b/comma.go
index ceba762..26666e1 100644
--- a/comma.go
+++ b/comma.go
@@ -13,7 +13,7 @@
 		v = 0 - v
 	}
 
-	parts := make([]string, 9)
+	parts := []string{"", "", "", "", "", "", "", "", ""}
 	j := len(parts) - 1
 
 	for v > 999 {