Consider adding explicit outside world nodes to interproc CFG

[d1m0]

Currently all paths in the interprocedural CFG start with nil and end with the last node in the body of the entry function to execute.
Additionally, interprocedural CFGs for now ignore external calls.

It might be easier to filter paths if we had explicit 'start', 'end' and 'external_call' nodes, with paths to them. This would allow clients to build analyses on top of interprocCFG that have their own special handling for external calls.

Alternatively, we could have a single Env node, that is both the begining, end and intermediate outside calls. This is a nicer/simpler way of modelling, but it makes all paths in the interproc CFG implicitly cyclic.

Not sure yet which is the better choice.