Fold instances of different size

[maramihali]

We'll use padding as the instances need to be of fixed size for protocol to work (size TBD, maybe 2^19)

[maramihali]

The following issue is a dependent this https://github.com/AztecProtocol/barretenberg/issues/844

[codygunton]

Note that this issue include handling multiple types of data where lengths can be inconsistent, including public inputs (even if instances are the same size, in fact).

[ledwards2225]

Support for this was implicit in the introduction of the overflow mechanism but we also don't plan to use this outside of that context