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/eth2_interop_keypairs/src/lib.rs

@@ -106,8 +106,8 @@ impl TryInto<Keypair> for YamlKeypair {
 }
 
 fn string_to_bytes(string: &str) -> Result<Vec<u8>, String> {
-    let string = if string.starts_with("0x") {
-        &string[2..]
+    let string = if let Some(stripped) = string.strip_prefix("0x") {
+        stripped
     } else {
         string
     };