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.

lighthouse/src/main.rs

@@ -200,7 +200,7 @@ fn run<E: EthSpec>(
 
     let debug_level = matches
         .value_of("debug-level")
-        .ok_or_else(|| "Expected --debug-level flag".to_string())?;
+        .ok_or("Expected --debug-level flag")?;
 
     let log_format = matches.value_of("log-format");