The notable places that are changed are PyEq, PyLt, and
compare/compareInternal.

The old codepaths are still in there (though thankfully
now with reduced duplication with cpython), since for anything
that defines a Python-level __lt__, it's better to call that
with our rewriting support, rather than calling it through
slot_tp_richcompare.

The control flow is kind of messy, since I don't think we
know what the right long-term organization is for these kinds of
things.  But basically, it's:
- if we can't rewrite, just call the C slot
- if we can rewrite, and we think calling tp_richcompare is profitable
  (ie it's not slot_tp_richcompare), we call that and emit a call to
  it in the patchpoint
- otherwise, we try calling the python attribute

Actual conversion of our attributes to tp_richcompare in the next commit.