The graphml reader no longer builds a list of edges/vertices during
parsing, whenever the graph uses canonical names.

These are redundant functions which can be trivially implemented as
loops.

This fixes ticket #131, and includes a 'fast' O(k) option, which
changes the vertex index ordering.

This is done by default in depth_first_search() in BGL, so we switch to
depth_first_visit().

This avoids a conflict with the overloaded tie() function.