Replacement for original tsort.

The old code suffers from a few defects:
- it does not even implement the standard optimal topological sort
algorithm. It's much slower.
- its longest cycle computation is completely bogus.

This is clean-slate code, that does implement the actual standard optimal
topological sort, together with a correct graph traversal to find longest
cycles.

It does also feature a `stable tsort' mode, where it uses a heap to yield
the least disturbed permutation of input nodes that satisfies the ordering
constraints (in particular, try tsort -f).

Thanks to the nature of the problem, the actual output won't exactly match
the old one, but it does pass the regression suite (and it is a topological
sorter).

Ok millert@

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