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.

testing/ef_tests/src/cases/operations.rs

@@ -304,10 +304,7 @@ impl<E: EthSpec, O: Operation<E>> LoadCase for Operations<E, O> {
 
 impl<E: EthSpec, O: Operation<E>> Case for Operations<E, O> {
     fn description(&self) -> String {
-        self.metadata
-            .description
-            .clone()
-            .unwrap_or_else(String::new)
+        self.metadata.description.clone().unwrap_or_default()
     }
 
     fn is_enabled_for_fork(fork_name: ForkName) -> bool {