Update random nullifier for split notes

[ConstanceBeguier]

To be secure against roadblock attacks, we update the process to obtain a random nullifier for split notes.
Now we have the following formula to evaluate nf_old

• for non split_notes, nf_old = Extract_P([PRF^{nfOrchard}_{nk}(rho_old) + psi_nf) mod q_P] NullifierK + cm_old)
• for split notes, nf_old = Extract_P([PRF^{nfOrchard}_{nk}(rho_old) + psi_nf) mod q_P] NullifierK + cm_old + NullifierL)
  where psi_nf is equal to
• psi_old for non split notes
• a random pallas Base element for split notes

The following constraints have been updated into the circuit

• nf_old = nf_old_pub for all notes
• derived_pk_d_old = pk_d_old for all notes
• if split_flag=0, then psi_old = psi_new

[PaulLaux]

Added minor comment.

Also, please remove comment on line 144 for bundle.rs