Add a module to the system, creating edges between this module and other modules as appropriate.
Modules are maps of the form {:ins #{...} :outs #{...}} where the set of ins are the input topics
and the set of outs are output topics

Returns an x-form that will transform another module into
the edges that go from the given module to the other module

Create all the edges that connect the given module to the system

Given a from-module and to-module, return the set of topics that
are members of the outs of the from-module and the ins of the to-module.
If the from-module emits :any and the to-module accepts :any, return #{:any}

A system with no modules

Returns an x-form that will transform another module into
the edges that go from the other module to the given module

Recreate the system, applying the given x-form to the nodes of the system