Results of a line-by-line comparison back to dictobject.c.

Results of a line-by-line comparison back to dictobject.c.

* set_merge() cannot assume that the table doesn't resize during iteration.

* convert some unnecessary tests to asserts -- they were necessary in
  dictobject.c because PyDict_Next() is a public function.  The same is
  not true for set_next().

* re-arrange the order of functions to more closely match the order
  in dictobject.c.  This makes it must easier to compare the two
  and ought to simplify any issues of maintaining both.