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Add the ability to customize the continuation prompt.

- Remove unused property `def_prompt`.
Formatting only: fix alignment.
Reformat help message for `:show` to fit into 80 cols again.
Whitespace only
Fix thunk leak in CoreM's CoreWriter
Update docs for "set prompt2"
Imrove Lint to check unfoldings
Fix panic on deriving a nullary typeclass (#7959)
Comments and white space only
Implement cardinality analysis

This major patch implements the cardinality analysis described
in our paper "Higher order cardinality analysis". It is joint
work with Ilya Sergey and Dimitrios Vytiniotis.

The basic is augment the absence-analysis part of the demand
analyser so that it can tell when something is used
	 never
	 at most once
 	 some other way

The "at most once" information is used
    a) to enable transformations, and
       in particular to identify one-shot lambdas
    b) to allow updates on thunks to be omitted.

There are two new flags, mainly there so you can do performance
comparisons:
    -fkill-absence   stops GHC doing absence analysis at all
    -fkill-one-shot  stops GHC spotting one-shot lambdas
                     and single-entry thunks

The big changes are:

* The Demand type is substantially refactored.  In particular
  the UseDmd is factored as follows
      data UseDmd
        = UCall Count UseDmd
        | UProd [MaybeUsed]
        | UHead
        | Used

      data MaybeUsed = Abs | Use Count UseDmd

      data Count = One | Many

  Notice that UCall recurses straight to UseDmd, whereas
  UProd goes via MaybeUsed.

  The "Count" embodies the "at most once" or "many" idea.

* The demand analyser itself was refactored a lot

* The previously ad-hoc stuff in the occurrence analyser for foldr and
  build goes away entirely.  Before if we had build (\cn -> ...x... )
  then the "\cn" was hackily made one-shot (by spotting 'build' as
  special.  That's essential to allow x to be inlined.  Now the
  occurrence analyser propagates info gotten from 'build's stricness
  signature (so build isn't special); and that strictness sig is
  in turn derived entirely automatically.  Much nicer!

* The ticky stuff is improved to count single-entry thunks separately.

One shortcoming is that there is no DEBUG way to spot if an
allegedly-single-entry thunk is acually entered more than once.  It
would not be hard to generate a bit of code to check for this, and it
would be reassuring.  But it's fiddly and I have not done it.

Despite all this fuss, the performance numbers are rather under-whelming.
See the paper for more discussion.

       nucleic2          -0.8%    -10.9%      0.10      0.10     +0.0%
         sphere          -0.7%     -1.5%      0.08      0.08     +0.0%
--------------------------------------------------------------------------------
            Min          -4.7%    -10.9%     -9.3%     -9.3%    -50.0%
            Max          -0.4%     +0.5%     +2.2%     +2.3%     +7.4%
 Geometric Mean          -0.8%     -0.2%     -1.3%     -1.3%     -1.8%

I don't quite know how much credence to place in the runtime changes,
but movement seems generally in the right direction.
Comments about the Name Cache
Wibbles (merg-os) to ticky-ticky
Add TyCon.checkRecTc, and use in in typeArity

This just formalises an abstraction we've been using anyway,
namely to expand "recursive" TyCons until we see them twice.
We weren't doing this in typeArity, and that inconsistency
was leading to a subsequent ASSERT failure, when compiling
Stream.hs, which has a newtype like this

   newtype Stream m a b = Stream (m (Either b (a, Stream m a b)))
Remove bogus-looking check, which was causing Trac #7894
Use checkRecTc to improve demand analysis slightly

We now look inside one layer of recursive types, thanks to
checkRecTc.  It does mean we need an additional environment
field, ae_rec_tc.

I also commented out the apparently over-conservative test
at coercions.  I'm not 100% sure I'm right here, but I can't
see why the simpler code will go wrong, so I'm going to suck
it and see.
Transfer strictness on trivial right-hand sides

See Note [Trivial right-hand sides]