Add convenience builders and graphviz label override

Merge

Add an explicit condition hierarchy

Fixes #5

Detect *all* cycles during topological sort

Previously the cycle detection was limited to detecting when we hit
a currently-being-visited node during a traversal.  So something like this would
be correctly found:

    A --> B --> C
          ^     |
          |     |
          +-----+

We start at the root (A), go to B, then to C, then to B, and detect that we're
still working on B and signal the error.

But this doesn't find all cycles, because we *start* at the root nodes, and if
a cycle doesn't have any outcropping branches we'll never reach it at all.  For
example:

    A --> B
    ^     |
    |     |
    +-----+

This graph has no roots, so we incorrectly ignore the cycle.

This patch fixes the problem by keeping a count of visited nodes and and making
sure it matches the digraph's size at the end.

Fixes #4

Bump version, update changelog

Add `arbitrary-vertex`

Make copy-digraph actually copy the data

Bump version

Bump version & changelog

Add `emptyp`, export `roots` and `leafs`