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.

crypto/bls/Cargo.toml

@@ -2,7 +2,7 @@
 name = "bls"
 version = "0.2.0"
 authors = ["Paul Hauner <paul@paulhauner.com>"]
-edition = "2018"
+edition = "2021"
 
 [dependencies]
 eth2_ssz = "0.4.1"

crypto/eth2_hashing/Cargo.toml

@@ -2,7 +2,7 @@
 name = "eth2_hashing"
 version = "0.2.0"
 authors = ["Paul Hauner <paul@paulhauner.com>"]
-edition = "2018"
+edition = "2021"
 license = "Apache-2.0"
 description = "Hashing primitives used in Ethereum 2.0"
 

crypto/eth2_key_derivation/Cargo.toml

@@ -2,7 +2,7 @@
 name = "eth2_key_derivation"
 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
 

crypto/eth2_key_derivation/src/derived_key.rs

@@ -148,8 +148,7 @@ fn parent_sk_to_lamport_pk(ikm: &[u8], index: u32) -> ZeroizeHash {
 
     lamports
         .iter()
-        .map(LamportSecretKey::iter_chunks)
-        .flatten()
+        .flat_map(LamportSecretKey::iter_chunks)
         .enumerate()
         .for_each(|(i, chunk)| {
             let mut hasher = Sha256::new();

crypto/eth2_keystore/Cargo.toml

@@ -2,7 +2,7 @@
 name = "eth2_keystore"
 version = "0.1.0"
 authors = ["Pawan Dhananjay <pawan@sigmaprime.io", "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
 

crypto/eth2_wallet/Cargo.toml

@@ -2,7 +2,7 @@
 name = "eth2_wallet"
 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