Add `OrchardFixedBasesFull::{generator, u}` methods

Using these in `OrchardFixedBases::{generator, u}` instead of the
`impl From<OrchardFixedBasesFull> for OrchardFixedBase` means we avoid
computing the Lagrange coefficients for the generator (which were then
immediately dropped).

This decreases proving time in the Action circuit by 53%.

src/circuit/gadget/ecc/chip/mul_fixed.rs

@@ -48,20 +48,14 @@ impl OrchardFixedBases {
     pub fn generator(self) -> pallas::Affine {
         match self {
             Self::ValueCommitV => constants::value_commit_v::generator(),
-            Self::Full(base) => {
-                let base: OrchardFixedBase = base.into();
-                base.generator
-            }
+            Self::Full(base) => base.generator(),
         }
     }
 
     pub fn u(self) -> Vec<WindowUs> {
         match self {
             Self::ValueCommitV => ValueCommitV::get().u_short.0.as_ref().to_vec(),
-            Self::Full(base) => {
-                let base: OrchardFixedBase = base.into();
-                base.u.0.as_ref().to_vec()
-            }
+            Self::Full(base) => base.u().0.as_ref().to_vec(),
         }
     }
 }

src/constants/load.rs

@@ -12,6 +12,28 @@ pub enum OrchardFixedBasesFull {
     SpendAuthG,
 }
 
+impl OrchardFixedBasesFull {
+    pub fn generator(&self) -> pallas::Affine {
+        match self {
+            OrchardFixedBasesFull::CommitIvkR => super::commit_ivk_r::generator(),
+            OrchardFixedBasesFull::NoteCommitR => super::note_commit_r::generator(),
+            OrchardFixedBasesFull::NullifierK => super::nullifier_k::generator(),
+            OrchardFixedBasesFull::ValueCommitR => super::value_commit_r::generator(),
+            OrchardFixedBasesFull::SpendAuthG => super::spend_auth_g::generator(),
+        }
+    }
+
+    pub fn u(&self) -> U {
+        match self {
+            OrchardFixedBasesFull::CommitIvkR => super::commit_ivk_r::U.into(),
+            OrchardFixedBasesFull::NoteCommitR => super::note_commit_r::U.into(),
+            OrchardFixedBasesFull::NullifierK => super::nullifier_k::U.into(),
+            OrchardFixedBasesFull::ValueCommitR => super::value_commit_r::U.into(),
+            OrchardFixedBasesFull::SpendAuthG => super::spend_auth_g::U.into(),
+        }
+    }
+}
+
 /// A fixed base to be used in scalar multiplication with a full-width scalar.
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub struct OrchardFixedBase {