use consts BFIELD_ZERO and BFIELD_ONE for traits Zero and One

Avoid Montgomery reduction for those known, frequently used constants.

twenty-first/src/shared_math/b_field_element.rs

@@ -403,24 +403,24 @@ impl FiniteField for BFieldElement {}
 impl Zero for BFieldElement {
     #[inline]
     fn zero() -> Self {
-        BFieldElement::new(0)
+        BFIELD_ZERO
     }
 
     #[inline]
     fn is_zero(&self) -> bool {
-        self.canonical_representation() == 0
+        self == &BFIELD_ZERO
     }
 }
 
 impl One for BFieldElement {
     #[inline]
     fn one() -> Self {
-        BFieldElement::new(1)
+        BFIELD_ONE
     }
 
     #[inline]
     fn is_one(&self) -> bool {
-        self.canonical_representation() == 1
+        self == &BFIELD_ONE
     }
 }