DData revision

[Simon Marlow]

> A revision of DData.
> 
> http://users.skynet.be/jyp/DData/doc
> http://users.skynet.be/jyp/DData/ddata.tar.gz
> 
> New sequences are added. I've tried to reflect settled
> issues in the code. If it is not so, tell me.

I like the new Seq.  Do we need DData.LambdaSeq too?

Does it bother anyone that the use of Seq here migh get confused with
the 'seq' function?  Similarly for Map.

Cheers,
	Simon