adding itoa_static and ft_nbrlen

srcs/ft_nbrlen.c

@@ -0,0 +1,16 @@
+
+#include "libft.h"
+
+// return length of int
+// eg. 123 -> 3, -123 -> 4
+
+size_t ft_nbrlen(int nbr)
+{
+    int len;
+
+    len = (nbr < 0) ? 2 : 1;
+    while (nbr /= 10)
+        len++;
+
+    return len;
+}