More small circuit optimisations

- Placing the Poseidon `state` columns after the `partial_sbox` column
  instead of before it causes them to line up with vast stretch of free
  space, enabling the pad-and-add region to be layed out there.

- Using the `Region::assign_advice_from_constant` API to initialise the
  Poseidon state removes fixed-column contention between that region and
  fixed-base scalar multiplication, enabling it to also be layed out
  within the free space.
  - If zcash/halo2#334 were implemented then
    this region would disappear.

- The overflow check in variable-base scalar mul is also moved into the
  columns with free space.

src/circuit.rs

@@ -241,8 +241,10 @@ impl plonk::Circuit<pallas::Base> for Circuit {
         let poseidon_config = PoseidonChip::configure(
             meta,
             poseidon::OrchardNullifier,
-            advices[5..8].try_into().unwrap(),
-            advices[8],
+            // We place the state columns after the partial_sbox column so that the
+            // pad-and-add region can be layed out more efficiently.
+            advices[6..9].try_into().unwrap(),
+            advices[5],
             rc_a,
             rc_b,
         );

src/circuit/gadget/ecc/chip/mul/overflow.rs

@@ -29,10 +29,12 @@ impl From<&EccConfig> for Config {
         Self {
             q_mul_overflow: ecc_config.q_mul_overflow,
             lookup_config: ecc_config.lookup_config.clone(),
+            // Use advice columns that don't conflict with the either the incomplete
+            // additions in fixed-base scalar mul, or the lookup range checks.
             advices: [
-                ecc_config.advices[0],
-                ecc_config.advices[1],
-                ecc_config.advices[2],
+                ecc_config.advices[6],
+                ecc_config.advices[7],
+                ecc_config.advices[8],
             ],
         }
     }

src/circuit/gadget/poseidon/pow5t3.rs

@@ -280,19 +280,12 @@ impl<F: FieldExt, S: Spec<F, WIDTH, 2>> PoseidonDuplexInstructions<F, S, WIDTH,
             || format!("initial state for domain {:?}", domain),
             |mut region| {
                 let mut load_state_word = |i: usize, value: F| {
-                    let var = region.assign_advice(
+                    let var = region.assign_advice_from_constant(
                         || format!("state_{}", i),
                         config.state[i],
                         0,
-                        || Ok(value),
+                        value,
                     )?;
-                    let fixed = region.assign_fixed(
-                        || format!("state_{}", i),
-                        config.rc_b[i],
-                        0,
-                        || Ok(value),
-                    )?;
-                    region.constrain_equal(var, fixed)?;
                     Ok(StateWord {
                         var,
                         value: Some(value),
@@ -754,6 +747,8 @@ mod tests {
                 meta.fixed_column(),
             ];
 
+            meta.enable_constant(rc_b[0]);
+
             Pow5T3Chip::configure(meta, OrchardNullifier, state, partial_sbox, rc_a, rc_b)
         }