Implement Incremental Automaton

[DavePearce]

After the work done on developing the IncrementalAutomatonMinimiser, it has become clear that closer integration with Automaton would be very helpful. See #19 for more.

Therefore, this issue proposes to rework Automaton in order to ensure all operations are incremental and do not do work proportional to the number of states. To make this happen requires:

1. Maintaining parent pointers for all states. This is necessary for all incremental algorithms and, in particular, enables incremental minimisation and compaction.
2. Resolving the issues of dangling cycles. That is, after a rewrite a set of states forming a cycle may not be garbage collected. Since the automaton is rooted, we can consider least common ancestors as one alternative approach.
3. Resolving the issue of making substitute() incremental. This algorithm is currently not incremental and I'm not sure how best to make it so.

These issues are really central to improving the overall performance of the compiler.