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Mention that the module sub-directory structure for .o and .hi files is created automatically by GHC

With the new Haddock patch, renamer errors can be duplicated;
so we want to bale out before doing the Haddock stuff if errors
are found.

(E.g test mod67 shows this up.)

This patch changes HscTypes.lookupIfaceByModule.  The problem was that
when compiling the 'base' package, we'd repeatedly reload GHC.Prim.
This is easily fixed by looking in the PIT too. A comment with
lookupIfaceByModule explains

This patch fixes Trac #909.  The problem was that when compiling 
the base package, the handling of wired-in things wasn't right;
in TcIface.tcWiredInTyCon it repeatedly loaded GHC.Base.hi into the
PIT, even though that was the very module it was compiling.

The main fix is by introducing TcIface.ifCheckWiredInThing.

But I did some minor refactoring as well.

This is a slight improvement over the patch sent by jamey@minilop.net,
we now do it properly if the source repo was a GHC tree on the local
filesystem too.

Merge post 6.6.

Comment Note [Take care] explains.

mkAtomicArgs is a mess.  A substantial rewrite of Simplify is needed.

This patch teaches SpecConstr about casts; see Note [SpecConstr for casts]

Note [Float coercions]
~~~~~~~~~~~~~~~~~~~~~~
When we find the binding
	x = e `cast` co
we'd like to transform it to
	x' = e
	x = x `cast` co		-- A trivial binding
There's a chance that e will be a constructor application or function, or something
like that, so moving the coerion to the usage site may well cancel the coersions
and lead to further optimisation.  Example:

     data family T a :: *
     data instance T Int = T Int

     foo :: Int -> Int -> Int
     foo m n = ...
        where
          x = T m
          go 0 = 0
          go n = case x of { T m -> go (n-m) }
		-- This case should optimise

The substitution used to carry "fragile" OccInfo to call sites via the
DoneId constructor of SimplEnv.SimplSR.  This was always a tricky thing 
to do, and for some time I've been removing the need for it.

Now at last I think we can nuke it altogether.  Hooray.

I did a full nonfib run, and got zero perf changes.

This is another attempt to fix the interaction between recursion and
RULES.  I just had it wrong before!  Now the significance of the
flag on IAmALoopBreaker is given in BasicTypes

  | IAmALoopBreaker	-- Used by the occurrence analyser to mark loop-breakers
			-- in a group of recursive definitions
	!RulesOnly	-- True <=> This loop breaker mentions the other binders
			--	    in its recursive group only in its RULES, not
			--	    in its rhs
			--  See OccurAnal Note [RulesOnly]