Fix contract on flow-ctx
Add a unit element for flow-ctx
Start using set-add/lattice-join! for efficiency reasons
Add a unit element for flow functions
Fix the contract for compute-flow-function
It isn't necessary to do the `env-refine`, it's a no-op

In a globally widened store we cannot throw away any information, it
must grow monotonically
Provide a way to count the callers and summaries sets