Automatically return All-bottom edge function for seeds

[ericbodden]

It seems that in IDE we always want to return the all-bottom edge function at initial seeds, if the input is zero. This should be added as a default to discourage mistakes on the side of the client analysis.

[johanneslerch]

This would be analog to the initialization of JumpFn with all-top edge functions. Nevertheless, it is different to the original algorithm as described in the IDE paper. There they use id edge functions for the seeds.

A third alternative may make even more sense: The current implementation allows clients to use facts other than ZERO at seeds. In addition, we could allow clients to provide custom values as well, while defaulting to Bottom. This would make analyses easier that require different start values for different kinds of seeds.

[ericbodden]

see #14 ...