change length or size specifiers in APIs from int to long

add array API (http://cr.yp.to/lib/array.html)

test/array.c

@@ -0,0 +1,13 @@
+#include <assert.h>
+#include "array.h"
+#include "byte.h"
+
+main() {
+  static array x,y;
+  array_cats(&x,"fnord");
+  array_cats(&y,"foobar");
+  array_cat(&x,&y);
+  array_fail(&y);
+  array_cat(&y,&x);
+  assert(byte_equal(x.p,11,"fnordfoobar"));
+}