Saving multiplications in the last round of optimized poseidon.

[krakhit]

In the Optimised poseidon we can use a small change that reduces unnecessary multiplications. This was noticed by @mickeyasa

Observation: In the last round: notice that after the S box operation, the full MDS matrix is used to generate all the elements in the state of width $t$. After that the round ends and the element with index state[1] is output as the digest.

Observation: There is no need to compute all the state elements in the MDS multiplication, in the last round.

Proposal: after the S box in the last round, only compute the element with index 1 in
state'= state^(transpose).MDS
i.e just compute state'[1]
since only the digest is needed, and other elements do not matter for the digest or another sequential hash.