Support program modules

[mmarx]

We want to have some kind of module, which is essentially a self-contained program with its own EDB and IDB predicates, separate from the rest. Invoking the module provides a mapping from predicates on the outside to EDB predicates for the module, and potentially from some IDB predicates of the module back to some predicates of the outside. The details of syntax and semantics are yet to be determined.