RFC: general sequences

[Ross Paterson]

A general implementation of sequences (based on work with Ralf Hinze)
can be found at

	http://www.soi.city.ac.uk/~ross/software/html/Data.Sequence.html
	http://www.soi.city.ac.uk/~ross/software/Data/Sequence.hs

Our experiments indicate that its performance is comparable to (and
sometimes better than) the best known persistent implementations(*).
Non-persistent implementations are typically faster, but you need to be
more careful when using them.

I'd like to propose this for base.

Comments welcome.

(*) or at least it will be when GHC's SPECIALIZE pragma is fixed for
    polymorphic specializations (SF bug #1019758).