Fix new clippy lints (#2036)

## Issue Addressed

NA

## Proposed Changes

Fixes new clippy lints in the whole project (mainly [manual_strip](https://rust-lang.github.io/rust-clippy/master/index.html#manual_strip) and [unnecessary_lazy_evaluations](https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_lazy_evaluations)). Furthermore, removes `to_string()` calls on literals when used with the `?`-operator.

account_manager/src/validator/create.rs

@@ -133,7 +133,7 @@ pub fn cli_run<T: EthSpec>(
     };
 
     let deposit_gwei = clap_utils::parse_optional(matches, DEPOSIT_GWEI_FLAG)?
-        .unwrap_or_else(|| spec.max_effective_balance);
+        .unwrap_or(spec.max_effective_balance);
     let count: Option<usize> = clap_utils::parse_optional(matches, COUNT_FLAG)?;
     let at_most: Option<usize> = clap_utils::parse_optional(matches, AT_MOST_FLAG)?;
 

account_manager/src/validator/exit.rs

@@ -125,7 +125,7 @@ async fn publish_voluntary_exit<E: EthSpec>(
     let keypair = load_voting_keypair(keystore_path, password_file_path, stdin_inputs)?;
 
     let epoch = get_current_epoch::<E>(genesis_data.genesis_time, spec)
-        .ok_or_else(|| "Failed to get current epoch. Please check your system time".to_string())?;
+        .ok_or("Failed to get current epoch. Please check your system time")?;
     let validator_index = get_validator_index_for_exit(client, &keypair.pk, epoch, spec).await?;
 
     let fork = get_beacon_state_fork(client).await?;
@@ -248,7 +248,7 @@ async fn get_beacon_state_fork(client: &BeaconNodeHttpClient) -> Result<Fork, St
         .get_beacon_states_fork(StateId::Head)
         .await
         .map_err(|e| format!("Failed to get get fork: {:?}", e))?
-        .ok_or_else(|| "Failed to get fork, state not found".to_string())?
+        .ok_or("Failed to get fork, state not found")?
         .data)
 }
 

account_manager/src/validator/slashing_protection.rs

@@ -49,7 +49,7 @@ pub fn cli_run<T: EthSpec>(
 
     let testnet_config = env
         .testnet
-        .ok_or_else(|| "Unable to get testnet configuration from the environment".to_string())?;
+        .ok_or("Unable to get testnet configuration from the environment")?;
 
     let genesis_validators_root = testnet_config
         .beacon_state::<T>()