Ignore compiled racket files
Fix erroneous referral of sub-lattices as chains

I originally though the product of a flat lattice and any other lattice was a
lattice with a series of disjoint chains. This is incorrect, it's a series of
disjoint sub-lattices.

The algorithm works the same regardless.
Use Racket's set library w/o import prefix

Previously, the CFA2 algorithm depended on the similar sets library which
shadowed Racket's default set library's bindings.

Now that the similar sets library has been replaced by the PartitionedSet
library, the CFA2 algorithm needn't prefix imported racket/set bindings.
Fix get-summaries and get-callers to respect information lattice

A summary may only be used when its open node subsumes the called-to node.

The added comments provide a detailed explanation of why this is the correct
behavior.