Proof multi-step computations composed of multiple interchange inputs & outputs

[pospi]

We should be able to do something like neighbour-hoods/rep_lang#57 but using separate calculations with statically stored results inbetween.

[mhuesch]

this is subsumed by #8, although that will only demonstrate multi-input/multi-output calculations over simple types.

however, we are already tracking user-defined types in neighbour-hoods/rep_lang#54, and implementing that, plus #8, should subsume the entire issue here. closing.