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

consensus/ssz/tests/tests.rs

@@ -2,7 +2,6 @@ use ethereum_types::H256;
 use ssz::{Decode, DecodeError, Encode};
 use ssz_derive::{Decode, Encode};
 
-#[allow(clippy::zero_prefixed_literal)]
 mod round_trip {
     use super::*;