polynomial: make Eq/PartialEq take leading zeros into account

Two polynomials are equal even if they have a different number of
leading zeros.

Putting this into its own commit rather than folding it into one of the
FieldVec commits, because this bug was present even before the FieldVec
stuff.

src/primitives/polynomial.rs

@@ -7,13 +7,21 @@ use core::{fmt, iter, ops, slice};
 use super::{ExtensionField, Field, FieldVec};
 
 /// A polynomial over some field.
-#[derive(PartialEq, Eq, Clone, Debug, Hash)]
+#[derive(Clone, Debug, Hash)]
 pub struct Polynomial<F> {
     /// The coefficients of the polynomial, in "little-endian" order.
     /// That is the constant term is at index 0.
     inner: FieldVec<F>,
 }
 
+impl<F: Field> PartialEq for Polynomial<F> {
+    fn eq(&self, other: &Self) -> bool {
+        self.inner[..self.degree()] == other.inner[..other.degree()]
+    }
+}
+
+impl<F: Field> Eq for Polynomial<F> {}
+
 impl<F: Field> Polynomial<F> {
     /// Determines whether the residue is representable, given the current
     /// compilation context.