change length or size specifiers in APIs from int to long

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

array/array_cat.c

@@ -0,0 +1,10 @@
+#include "array.h"
+#include "byte.h"
+
+void array_cat(array* to,const array* const from) {
+  if (from->allocated<0) {
+    array_fail(to);
+    return;
+  }
+  return array_catb(to,from->p,from->initialized);
+}