[keys] Consider assuming a key's point is always non-infinite

[nventuro]

Given #7514 and #7515 we may be able to assume that keys are not infinite. If so, we can reduce the PublicKeys serialization from 12 elements (4 x,y,inf tuples) to 8 (just the x,y coords). This in turn results in a smaller hash, reducing gate counts when reading keys via e.g. #7523.

Simple testing shows a decrease of ~1.4k gates by removing the booleans, so ~2.8k if we're reading keys of two accounts. It may be that these gains are not as big if poseidon performance improves however.

[nventuro]

Note that this would also reduce storage costs.