Add ovm.py, a subset of byterun which runs gold/fibonacci.py.

This is to plan what needs to go in our new VM.  In particular, we want
more, not the BREAK_LOOP / block stack stuff which is implemented in
CPython (and in the current version of ovm.py.)

- Add ovm_codegen.py and hook it up to opy_main.py and test.sh.  It
  doesn't work yet, but it's useful because it makes note of all the AST
  node types we have to handle.

- Stub out a 'cfg' action.  (However, I realized that we don't want to
  use Python's control flow graph as-is.)