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You can say :set -fno-print-bind-result in GHCi to disable this behaviour.
Addresses #887

Right now we can only manage to add a source code link for the module,
but that's better than nothing.

I had to put the list of core packages in a Makefile variable,
$(CorePackages), so we'll have to be careful to keep this up to date.
(I could have slurped it out of libraries/core-packages with $(shell),
but that's ugly and really slow on Windows).

There are a couple of new tweakables: CorePackageSourceURL and
ExtraPackageSourceURL in config.mk.in, set these to the appropriate
patterns for generating source links.

(when we merge this patch onto the HEAD we'll have to tweak these
settings).

Unfortunately it still doesn't work for all the modules, because
modules compiled without -cpp don't get any #line directives.  More
hackery required...

I'm not sure if this is the example that was intended, but it's at least
now consistent.

The newtype-deriving mechanism generates a HsSyn case expression looking
like this
	case (d `cast` co) of { ... }
That is, the case expression scrutinises a dictionary.  This is 
otherwise never seen in HsSyn, and it made the desugarer
(Check.get_unused_cons) crash in tcTyConAppTyCon.

It would really be better to generate Core in TcInstDecls (the newtype
deriving part) but I'm not going to do that today.  Instead, I made
Check.get_unused_cons a bit more robust.

Audrey tried to fix this over the weekend, but her fix was, alas, utterly
bogus, which caused mysterious failures later.  I completely undid this
change.

Anyway it should work now!

Comment-only: Fix a typo, and note that the PredTy case on SplitTyConApp_maybe was added as a kluge.

such that this can compile again:

    newtype Moose = MkMoose () deriving (Eq, Ord)

The newtype deriving mechanism is much trickier to support than it
seems at first.  Kevin didn't get it quite right when moving to FC,
and I ended up re-writing quite a bit of it.  

I think it's right now, but I have not yet tested it thoroughly.

This patch enables argToPat to look through let expressions

e.g.	f (let v = rhs in \y -> ...v...)

Here we can specialise for f (\y -> ...) because the rule-matcher will
look through the let.

This fixes a typo -- a missing prime in tcConPat.  

The test is gadt18.

While modifying TcPat I also trimmed imports, fixed non-exhaustive
patterns, and improved tracing.

In ByteCodeGen, correctly passthru AnnCast in all relevant places, so the previous band-aid fix is no longer needed.