This patch fixes a bad bug in the specialiser, which showed up as
Trac #13429.  When specialising an imported DFun, the specialiser could
generate a recusive loop where none existed in the original program.

It's all rather tricky, and I've documented it at some length in
   Note [Avoiding loops]

We'd encoutered exactly this before (Trac #3591) but I had failed
to realise that the very same thing could happen for /imported/
DFuns.

I did quite a bit of refactoring.

The compiler seems to get a tiny bit faster on
   deriving/perf/T10858
but almost all the gain had occurred before now; this
patch just pushed it over the line.