Adopt idiomatic code suggestions.

src/bls12_381/fq.rs

@@ -334,7 +334,7 @@ impl PrimeFieldRepr for FqRepr {
 
         while n >= 64 {
             let mut t = 0;
-            for i in self.0.iter_mut() {
+            for i in &mut self.0 {
                 ::std::mem::swap(&mut t, i);
             }
             n -= 64;

src/bls12_381/fr.rs

@@ -170,7 +170,7 @@ impl PrimeFieldRepr for FrRepr {
 
         while n >= 64 {
             let mut t = 0;
-            for i in self.0.iter_mut() {
+            for i in &mut self.0 {
                 ::std::mem::swap(&mut t, i);
             }
             n -= 64;

src/lib.rs

@@ -482,7 +482,7 @@ pub trait PrimeField: Field
     /// Interpret a string of numbers as a (congruent) prime field element.
     /// Does not accept unnecessary leading zeroes or a blank string.
     fn from_str(s: &str) -> Option<Self> {
-        if s.len() == 0 {
+        if s.is_empty() {
             return None;
         }