Update to Rust 1.59 and 2021 edition (#3038)

## Proposed Changes

Lots of lint updates related to `flat_map`, `unwrap_or_else` and string patterns. I did a little more creative refactoring in the op pool, but otherwise followed Clippy's suggestions.

## Additional Info

We need this PR to unblock CI.

consensus/tree_hash/src/merkle_hasher.rs

@@ -376,8 +376,8 @@ mod test {
     fn compare_with_reference(leaves: &[Hash256], depth: usize) {
         let reference_bytes = leaves
             .iter()
-            .map(|hash| hash.as_bytes().to_vec())
-            .flatten()
+            .flat_map(|hash| hash.as_bytes())
+            .copied()
             .collect::<Vec<_>>();
 
         let reference_root = merkleize_padded(&reference_bytes, 1 << (depth - 1));