Implement list fusion using streams instead of foldr/build

[simonpj]

We'd like to try using the stream-fusion idea of Don Stewart, Duncan Coutts and Roman Leshchinskiy, and replace the (somewhat fragile) foldr/build stuff.

See #876.

[dons]

This task has its own website now:

 http://www.cse.unsw.edu.au/~dons/streams.html.

[guest]

it will be great to see this included in ghc base if this work is already completed

[duncan]

It's not done yet. There's a paper on it but we still have issues with complex list comprehensions.

The new streams stuff will be included in the new bytestring-1.0 package though (as you know, the base package is being split up and ByteString? will be in its own package).

[dons]

We should also rerun the benchmarks in the light of the changes that made it into head since April when the fusion library was last benchmarked. There's no that much more to do here, to finish the job.

[dons]

This library has been written and released,

 http://hackage.haskell.org/cgi-bin/hackage-scripts/package/stream-fusion

To replace GHC's existing list fusion, we'll need to work out how concatMap is optimised, and push the desugaring for list comprehensions into GHC.

For now though, users can just use the library, and avoid list comprehensions and [n .. m] (use enumFromTo).