Merkle proof input signals can be reduced in size

[Stentonian]

e.g. https://github.com/silversixpence-crypto/zk-proof-of-assets/blob/3092dda0558ee8fb5ee96b560bd29915e8e00d30/tests/2_sigs/layer_two_input.json#L394-L420

Only 1 bit is needed for each element but a whole field element is used. We can squash the array to a single binary value.