Address clippy lints, panic in ssz_derive on overflow (#1714)

## Issue Addressed

NA

## Proposed Changes

- Panic or return error if we overflow `usize` in SSZ decoding/encoding derive macros.
  - I claim that the panics can only be triggered by a faulty type definition in lighthouse, they cannot be triggered externally on a validly defined struct.
- Use `Ordering` instead of some `if` statements, as demanded by clippy.
- Remove some old clippy `allow` that seem to no longer be required.
- Add comments to interesting clippy statements that we're going to continue to ignore.
- Create #1713

## Additional Info

NA

validator_client/src/initialized_validators.rs

@@ -414,7 +414,6 @@ impl InitializedValidators {
     /// validator will be removed from `self.validators`.
     ///
     /// Saves the `ValidatorDefinitions` to file, even if no definitions were changed.
-    #[allow(dead_code)] // Will be used once VC API is enabled.
     pub async fn set_validator_status(
         &mut self,
         voting_public_key: &PublicKey,