Optimise layout of witnessed Sinsemilla message pieces

The Sinsemilla chip witnesses message pieces in individual regions, and
then copies them into the `hash_piece` region to initialize the running
sum. Previously these occured in the same column, but we can reduce the
utilized rows of the Action circuit by moving these into a less-used
column.

If https://github.com/zcash/halo2/issues/334 is implemented, this change
would be unnecessary, as the witnessed message piece regions would never
be assigned into the circuit.

src/circuit.rs

@@ -252,6 +252,7 @@ impl plonk::Circuit<pallas::Base> for Circuit {
             let sinsemilla_config_1 = SinsemillaChip::configure(
                 meta,
                 advices[..5].try_into().unwrap(),
+                advices[6],
                 lookup,
                 range_check.clone(),
             );
@@ -268,6 +269,7 @@ impl plonk::Circuit<pallas::Base> for Circuit {
             let sinsemilla_config_2 = SinsemillaChip::configure(
                 meta,
                 advices[5..].try_into().unwrap(),
+                advices[7],
                 lookup,
                 range_check,
             );

src/circuit/gadget/sinsemilla.rs

@@ -484,12 +484,14 @@ mod tests {
             let config1 = SinsemillaChip::configure(
                 meta,
                 advices[..5].try_into().unwrap(),
+                advices[2],
                 lookup,
                 range_check.clone(),
             );
             let config2 = SinsemillaChip::configure(
                 meta,
                 advices[5..].try_into().unwrap(),
+                advices[7],
                 lookup,
                 range_check,
             );

src/circuit/gadget/sinsemilla/chip.rs

@@ -53,6 +53,9 @@ pub struct SinsemillaConfig {
     /// Advice column used to store the $\lambda_2$ intermediate value at each
     /// iteration.
     lambda_2: Column<Advice>,
+    /// Advice column used to witness message pieces. This may or may not be the same
+    /// column as `bits`.
+    witness_pieces: Column<Advice>,
     /// The lookup table where $(\mathsf{idx}, x_p, y_p)$ are loaded for the $2^K$
     /// generators of the Sinsemilla hash.
     pub(super) generator_table: GeneratorTableConfig,
@@ -106,6 +109,7 @@ impl SinsemillaChip {
     pub fn configure(
         meta: &mut ConstraintSystem<pallas::Base>,
         advices: [Column<Advice>; 5],
+        witness_pieces: Column<Advice>,
         lookup: (Column<Fixed>, Column<Fixed>, Column<Fixed>),
         range_check: LookupRangeCheckConfig<pallas::Base, { sinsemilla::K }>,
     ) -> <Self as Chip<pallas::Base>>::Config {
@@ -118,6 +122,7 @@ impl SinsemillaChip {
             bits: advices[2],
             lambda_1: advices[3],
             lambda_2: advices[4],
+            witness_pieces,
             generator_table: GeneratorTableConfig {
                 table_idx: lookup.0,
                 table_x: lookup.1,
@@ -241,7 +246,7 @@ impl SinsemillaInstructions<pallas::Affine, { sinsemilla::K }, { sinsemilla::C }
             |mut region| {
                 region.assign_advice(
                     || "witness message piece",
-                    config.bits,
+                    config.witness_pieces,
                     0,
                     || field_elem.ok_or(Error::SynthesisError),
                 )

src/circuit/gadget/sinsemilla/commit_ivk.rs

@@ -702,6 +702,7 @@ mod tests {
                 let sinsemilla_config = SinsemillaChip::configure(
                     meta,
                     advices[..5].try_into().unwrap(),
+                    advices[2],
                     lookup,
                     range_check.clone(),
                 );

src/circuit/gadget/sinsemilla/merkle.rs

@@ -203,6 +203,7 @@ pub mod tests {
             let sinsemilla_config_1 = SinsemillaChip::configure(
                 meta,
                 advices[5..].try_into().unwrap(),
+                advices[7],
                 lookup,
                 range_check.clone(),
             );
@@ -211,6 +212,7 @@ pub mod tests {
             let sinsemilla_config_2 = SinsemillaChip::configure(
                 meta,
                 advices[..5].try_into().unwrap(),
+                advices[2],
                 lookup,
                 range_check,
             );

src/circuit/gadget/sinsemilla/note_commit.rs

@@ -1364,6 +1364,7 @@ mod tests {
                 let sinsemilla_config = SinsemillaChip::configure(
                     meta,
                     advices[..5].try_into().unwrap(),
+                    advices[2],
                     lookup,
                     range_check.clone(),
                 );