Circuit: Update note_commit to take into account asset

[ConstanceBeguier]

In the circuit, we update note_commit to take into account asset.
Previously, note_commit returns cm = hash(Q_ZEC, msg) + [rcm]R.
Now, note_commit returns

• cm = hash(Q_ZEC, msg) + [rcm]R for ZEC note
• cm = hash(Q_ZSA, msg || asset) + [rcm]R for ZSA note

We now evaluate note_commit with the following steps

1. evaluate hash_zec = hash(Q_ZEC, msg)
2. evaluate hash_zsa = hash(Q_ZSA, msg || asset)
3. select hash = hash_zec if is_native_asset
   = hash_zsa otherwise
4. evaluate cm = hash + [rcm]R
5. check some constraints on msg and asset and their decompositions
6. return cm

The following modifications are required to update note_commit:

• add a is_native_asset witness (and check that it is a boolean and its value is correct according to asset)
• add a MUX chip to evaluate a multiplexer on Pallas points

Warning: we increased the size of the Orchard circuit !

[ConstanceBeguier]

I just rebased on zsa1 branch

[PaulLaux]

added some minor comments. As discussed, let's wait for today's meeting before the final approve.

[PaulLaux]

I don't like the duplication between x_cell and y_cell but don't see an easy way to remove it.

[ConstanceBeguier]

I just rebased on zsa1

[PaulLaux]

Let's merge it.