Circuit: Update value_commit_orchard to take into account asset (deprecated)

[ConstanceBeguier]

In the circuit, we update value_commit_orchard to take into account asset.
Previously, value_commit_orchard returns cv_net = [v_net] ValueCommitV + [rcv] ValueCommitR..
Now, value_commit_orchard returns cv_net = [v_net] asset + [rcv] ValueCommitR.
ValueCommitV and ValueCommitR are constants

[what-the-diff]

• Remove asset_old
• Add asset to the circuit struct and assign it with spend note's asset in constructor function
• Constrain cv_net = ValueCommit^Orchard(rcv, v_net) instead of cv_new = ValueCommit^Orchard(rcv, v_new). This is because we want to verify that the value commitment for a given output note (cv) equals its corresponding public input (cv') which is not necessarily equal to the new value commitment generated by this transaction but rather an old one from previous transactions.
• Change proof size accordingly: 4992 -> 5024; 7264 -> 7296
• Change the value commitment to be a point on curve instead of an affine
• Add tests for new circuit

[ConstanceBeguier]

Deprecated in favor of #50