Arrabbiata: setup phase

[dannywillems]

A setup phase is required to set the circuit layout up, and to initialize the transcripts.

A first iteration happened here, introducing the concept of "indexed relation".
On top of it, the setup phrase should account for:

• setting up the selectors of the circuit. The structure "IndexedRelation" should keep track of the commitments to the selectors for each circuit, and the initial state of the transcripts should include them.