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