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This egregious omission led to Trac #1128.

This patch improves prepareRhs, so that it deals better with casts.

We want to deal well cases like this
	v = (f e1 `cast` co) e2
Here we want to make e1,e2 trivial and get
	x1 = e1; x2 = e2; v = (f x1 `cast` co) v2

This really happens in parser libraries, which wrap functions in newtypes.

See Note [Casts and lambdas] in SimplUtils.  I found this transformation
when staring at some cast-heavy code generated by
Language.Haskell.Lexer.hs in the haskell-src library.

The basic transformation is this:
	(\x. e `cast` g1)  -->  (\x.e) `cast` (tx -> g1)
where x:tx.

This patch clears up a long-standing wart.   For some time it's been the
case that 
	the RHS of a non-recursive let can be unlifed iff 
	the RHS is ok-for-speculation

This patch extends the invariant to the argument of an App, and 
establishes it by the smart constructors mkDsApp, mkDsApps in the desugarer.

Once established, it should be maintained by the optimiser.

This tides up some awkward cases, notably in exprIsHNF, and I think it
fixes a outright strictness bug in Simplify.prepareRhs.

At the moment GHC really does very little simplification of coercions.
This patch improves matters, but it's still not great, especially when
you have chains linked together with 'trans'. 

I'm also concerned that I have not yet implemented the 'leftc' and 'rightc'
coercions we added to the paper. 

But at least things are better than they were.  In particular
	g `trans` sym g
now cancels to give the identity.

Fixes bug exposed by compiling base w/ -fasm on Windows

Moving functionality that was previously in the nightly build scripts
into the top-level Makefile, so it's easier to use from BuildBot.

In the generated code for case-of-variable, test the tag of the
scrutinee closure and only enter if it is unevaluated.  Also turn
*off* vectored returns.

This version does all its work in a subdirectory (no more destructive
'make distclean' in the current build tree), and results in the exact
-src.tar.bz2 distributions that we will put up for download.

This is only turned on with -O, and probably won't make much
difference at the moment, but it will be important for semi-tagging.

In the generated code for case-of-variable, test the tag of the
scrutinee closure and only enter if it is unevaluated.  Also turn
*off* vectored returns.

Only affects -fasm: gcc makes its own decisions about jump tables

Comparison of literal with narrowed/widened operand: perform
the comparison at a different width, as long as the literal is
within range.  We only do this on x86/x86_64 at the moment, where
we have comparisons at different sizes available.