All Simon's recent tuning changes.  Rough summary follows:

* Fix Kevin Atkinson's cant-find-instance bug.  Turns out that Rename.slurpSourceRefs
  needs to repeatedly call getImportedInstDecls, and then go back to slurping
  source-refs.  Comments with Rename.slurpSourceRefs.

* Add a case to Simplify.mkDupableAlt for the quite-common case where there's
  a very simple alternative, in which case there's no point in creating a
  join-point binding.

* Fix CoreUtils.exprOkForSpeculation so that it returns True of (==# a# b#).
  This lack meant that
	case ==# a# b# of { True -> x; False -> x }
  was not simplifying

* Make float-out dump bindings at the top of a function argument, as
  at the top of a let(rec) rhs.  See notes with FloatOut.floatRhs

* Make the ArgOf case of mkDupableAlt generate a OneShot lambda.
  This gave a noticeable boost to spectral/boyer2


* Reduce the number of coerces, using worker/wrapper stuff.
  The main idea is in WwLib.mkWWcoerce.  The gloss is that we must do
  the w/w split even for small non-recursive things.  See notes with
  WorkWrap.tryWw.

* This further complicated getWorkerId, so I finally bit the bullet and
  make the workerInfo field of the IdInfo work properly, including
  under substitutions.  Death to getWorkerId.  Kevin Glynn will be happy.

* Make all lambdas over realWorldStatePrimTy
  into one-shot lambdas.  This is a GROSS HACK.

* Also make the occurrence analyser aware of one-shot lambdas.

* Make various Prelude things into INLINE, so that foldr doesn't
  get inlined in their body, so that the caller gets the benefit
  of fusion.  Notably in PrelArr.lhs.