change length or size specifiers in APIs from int to long

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

array/array_length.c

@@ -0,0 +1,6 @@
+#include "array.h"
+
+int64 array_length(const array* const x,uint64 membersize) {
+  if (x->allocated<0) return -1;
+  return x->initialized/membersize;
+}