FWIW, I've wrapped up the key parts that I wanted to get done here
(co_localplusnames/kinds, MAKE_CELL, eliminate unused fast local
for arg cells).  I'm leaving this open for now as there are a few
things I didn't do that seem part of the original intention of this
issue:

* fully interleave the cells with the locals in their "natural" order
  (rather than only interleaving arg cells)
* update the compiler to track names/kinds rather than computing
  them after the fact during the assembler step
  (this will allow us to remove a decent amount of code)
* track the specific arg kinds in localspluskinds
  (this should allow us to make _PyEval_MakeFrameVector() simpler
  and a bit more efficient)