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/fork_choice/Cargo.toml

@@ -2,7 +2,7 @@
 name = "fork_choice"
 version = "0.1.0"
 authors = ["Paul Hauner <paul@paulhauner.com>"]
-edition = "2018"
+edition = "2021"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 

consensus/fork_choice/src/fork_choice.rs

@@ -219,7 +219,7 @@ fn dequeue_attestations(
         queued_attestations
             .iter()
             .position(|a| a.slot >= current_slot)
-            .unwrap_or_else(|| queued_attestations.len()),
+            .unwrap_or(queued_attestations.len()),
     );
 
     std::mem::replace(queued_attestations, remaining)

consensus/fork_choice/tests/tests.rs

@@ -613,7 +613,7 @@ fn justified_balances() {
 }
 
 macro_rules! assert_invalid_block {
-    ($err: tt, $($error: pat) |+ $( if $guard: expr )?) => {
+    ($err: tt, $($error: pat_param) |+ $( if $guard: expr )?) => {
         assert!(
             matches!(
                 $err,
@@ -719,7 +719,7 @@ fn invalid_block_finalized_descendant() {
 }
 
 macro_rules! assert_invalid_attestation {
-    ($err: tt, $($error: pat) |+ $( if $guard: expr )?) => {
+    ($err: tt, $($error: pat_param) |+ $( if $guard: expr )?) => {
         assert!(
             matches!(
                 $err,