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/types/src/beacon_state/tree_hash_cache.rs

@@ -1,5 +1,5 @@
 #![allow(clippy::integer_arithmetic)]
-#![allow(clippy::disallowed_method)]
+#![allow(clippy::disallowed_methods)]
 #![allow(clippy::indexing_slicing)]
 
 use super::Error;

consensus/types/src/lib.rs

@@ -7,7 +7,7 @@
     not(test),
     deny(
         clippy::integer_arithmetic,
-        clippy::disallowed_method,
+        clippy::disallowed_methods,
         clippy::indexing_slicing
     )
 )]