Replace SCC algorithm of containers

[Iavor Diatchki]

Hello,
Some time ago I implemented an algorithm that computes the strongly
connected components of a graph that is considerably faster than the
code in the containers package, at least for larger graphs.  This has
been available for a while in a separate package on hackage
(GraphSCC).  I propose that we merge the code into containers, and
simply replace the relevant functions.   Any objections?
-Iavor