Add ruleset rewriter tests

GraphSubst tests are insufficient, as we need to test all the renaming
malarky in the matching process as well.
Remove redundant normalisation calls
Properly remove edges in target graph

When substituting into a graph, we need to make sure all edges in the
match are removed; previously, we assumed that edges would be removed
when the vertices were.  This does not work when there was a bare wire
in the pattern graph, though.
Add (commented out) tests that fail for issue #108