Consistent mechanism for indicating kernel circuits from ACIR

[ledwards2225]

In the IVC setting, it must be known whether a circuit is a kernel or an app in order to properly perform the databus consistency checks. At some point I was using the presence of calldata acir constraints as an indicator (since apps only have return_data). This works in some cases but not in others. Another option is the presence of ivc_recursion_constraints in the constraint system. This is likely the better choice but either could potentially work. Either way, find a way to set this once and for all in a location sufficiently upstream so as to tricker all of the relevant logic.