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.

common/validator_dir/src/builder.rs

@@ -138,7 +138,7 @@ impl<'a> Builder<'a> {
     pub fn build(self) -> Result<ValidatorDir, Error> {
         let (voting_keystore, voting_password) = self
             .voting_keystore
-            .ok_or_else(|| Error::UninitializedVotingKeystore)?;
+            .ok_or(Error::UninitializedVotingKeystore)?;
 
         let dir = self
             .base_validators_dir

common/validator_dir/src/validator_dir.rs

@@ -167,8 +167,8 @@ impl ValidatorDir {
             .map_err(Error::UnableToReadDepositData)
             .and_then(|hex_bytes| {
                 let hex = std::str::from_utf8(&hex_bytes).map_err(|_| Error::DepositDataNotUtf8)?;
-                if hex.starts_with("0x") {
-                    hex::decode(&hex[2..]).map_err(Error::DepositDataInvalidHex)
+                if let Some(stripped) = hex.strip_prefix("0x") {
+                    hex::decode(stripped).map_err(Error::DepositDataInvalidHex)
                 } else {
                     Err(Error::DepositDataMissing0xPrefix)
                 }