Fix slow full_subgraph.

`AnnotatedGraph.full_subgraph` currently iterates over *all* graph edges, which is inefficient for large graphs.  In particular, that makes `strongly_connected_components` noticeably slow for graphs of more than a few thousand vertices.

Fixes #24.

This is really just a stopgap fix;  the correct fix is to get the edge information directly from the `strongly_connected_components` algorithm, and add a simple `subgraph` operation that takes the vertices *and* the edges of the subgraph as input.