Of all uses we have for Poseidon2 (Merkle Tree, Sponge absorption,
Sponge squeeze), none use the full 8 field elements of the output.
Sponge absorption uses only the final 4, and Merkle Tree and Sponge
squeeze uses only the initial 4.
This change introduces a switch for the user to select what parts of the
final state to output (first half, second half, all of it or none of
it).
---------
Co-authored-by: chriseth <chriseth.github@gmail.com>
bb3afd87d0