Handle inserting from rdcarray into itself

* This self-insertion has the same kind of problem as overlapping ranges in
  memcpy, the act of inserting items can affect the input range by shifting
  things around. For inserting a single object we just copy it, for inserting a
  range we duplicate the whole array and then do the insert from the old range
  (and destruct it).
* Clearly this is not the most efficient implementation, a better solution would
  be to append onto the existing array (potentially not even reallocating then)
  and doing a rotate/shift in place.