It would be useful to have complexity information in the documentation for functions

It would be handy for the documentation to confirm the expected time and space complexity of the functions in `fgl`. Given that all the functions operate over instances of `Graph` (and friends) rather than concrete types these bounds might have to be given in terms of the number of calls the `Graph`'s member functions, or given in terms of an idealized implementation of `Graph`.

This is particularly useful given the relative complexity of determining this information in Haskell.