RFC: general sequences

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).