Do not destruct rest arguments for __send__

Formerly, `__send__(*args)` modified `args` with `Array#shift`.
This bug affects optcarrot.

This changeset avoids the array destruction by using
`args = args[1, len-1]`.

src/array.c

@@ -808,6 +808,13 @@ ary_subseq(mrb_state *mrb, struct RArray *a, mrb_int beg, mrb_int len)
   return mrb_obj_value(b);
 }
 
+mrb_value
+mrb_ary_subseq(mrb_state *mrb, mrb_value ary, mrb_int beg, mrb_int len)
+{
+  struct RArray *a = mrb_ary_ptr(ary);
+  return ary_subseq(mrb, a, beg, len);
+}
+
 static mrb_int
 aget_index(mrb_state *mrb, mrb_value index)
 {