--------------------------
	Tidy up arity propagation
	--------------------------

Due to excessive complexity, correct arity information was getting
lost on the way to interface files.  As a result, a function that had
CPR info __S SLm (say), was getting arity 0, and this confused the (old)
CPR analyser ("lub of function and HasCPR").

I hope this fixes the above error (which showed up somewhere in
compiling Edison), but I'm going to commit it right now anyway.
Meanwhile I'll recompile Edison too.


Details
~~~~~~~
Digging out the rather obscure cause made me tidy up the CgInfo stuff.
The story is now

* The CgInfo field of an Id gets attached to the Id *only* in
  the TypeEnv of the ModuleDetails, during CoreTidy.

  This ModuleDetails stuff is used
	a) to generate the interface file
	b) to import into other modules in GHCi

* No CgInfo field is in the CoreBindings which are passed
  downsteam to CorePrep and thence CodeGen.  Quite right too...
  it's the downstream stuff that *generates* the CgInfo.

* But the Arity field *is* now passed on through CoreTidy
  (like strictness info) since it is usefully used by CorePrep.

* On the way I simplified the ArityInfo field of an IdInfo
  to simply
		Arity
  instead of
		Maybe Arity