fix sha256 warnings

src/sha256.rs

@@ -60,7 +60,7 @@ impl<Scalar: PrimeField> Circuit<Scalar> for MyCircuit {
         // Vec of None (indicating that the value of each bit is unknown).
         let bit_values = if let Some(preimage) = self.preimage {
             preimage
-                .into_iter()
+                .iter()
                 .map(|byte| (0..8).map(move |i| (byte >> i) & 1u8 == 1u8))
                 .flatten()
                 .map(|b| Some(b))
@@ -89,7 +89,7 @@ impl<Scalar: PrimeField> Circuit<Scalar> for MyCircuit {
 }
 
 fn main() {
-    use std::time::{Duration, Instant};
+    use std::time::Instant;
 
     let start = Instant::now();
     println!("Starting...");