Fix public key aggregation

[nulltea]

Adds doubling during public key aggregation and uses its result when the current accumulator equals the following public key to be summed with. Without this fix the case of such an event will result in failed proof generation.

However, the probability of such an event occurrence is quite low, so we should consider whether we want to incur the cost of performing 512 additional doubling and selection steps in the circuit. The cost in terms of advice cells is demonstrated below:

Before:

Gate Chip | Phase 0: 13062777 advice cells
Total 2063 fixed cells
Total range check advice cells to lookup per phase: [1328580, 0, 0]

After:

Gate Chip | Phase 0: 14383737 advice cells
Total 2063 fixed cells
Total range check advice cells to lookup per phase: [1531332, 0, 0]

Note: currently, it is still possible to use k=21 in the main circuit and k=23 for the aggregation step.