There still an issue with inferring for a given function call /
constant reference where that thing is defined.

We're currently generating the forest of references from the call
graph by calculating a spanning forest.  There's a problem with that.
Consider the following map of call-sites:

    {
      a: [b]
      b: []
    }

The following is a spanning tree:

    {
      a: []
      b: []
    }

Here it looks like `a` doesn't call `b`.


We should also store all the call-sites.  These will end up as leaves
in the call tree.


    {
      "K": {},
      "A": {},
      "f": {
        "f": "__cycle__"
      },
      "A.f": {
        "K.g": {}
      }
    }


Write graphviz output.