Fix Field::random method declarations.

bellman/src/groth16/tests/dummy_engine.rs

@@ -22,7 +22,7 @@ impl fmt::Display for Fr {
 }
 
 impl Field for Fr {
-    fn random<R: RngCore>(rng: &mut R) -> Self {
+    fn random<R: RngCore + ?std::marker::Sized>(rng: &mut R) -> Self {
         Fr(Wrapping(rng.next_u32()) % MODULUS_R)
     }
 

zcash_primitives/src/jubjub/fs.rs

@@ -317,7 +317,7 @@ impl PrimeField for Fs {
 }
 
 impl Field for Fs {
-    fn random<R: RngCore>(rng: &mut R) -> Self {
+    fn random<R: RngCore + ?std::marker::Sized>(rng: &mut R) -> Self {
         loop {
             let mut tmp = {
                 let mut repr = [0u64; 4];