[Circuit monad] Shared context

[mitschabaude]

Instead of instantiating subcircuits with a new Context F -- which determines the numbering of variables --, we pass the current context down to subcircuits

Thanks to this, in the final list of constraints we now have distinct indices for distinct vars, which seems important

[gio54321]

lgtm!
I agree, this is a cleaner way to structure things and does not change much the proofs