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

beacon_node/src/config.rs

@@ -21,7 +21,6 @@ use types::{ChainSpec, Checkpoint, Epoch, EthSpec, Hash256, GRAFFITI_BYTES_LEN};
 /// The output of this function depends primarily upon the given `cli_args`, however it's behaviour
 /// may be influenced by other external services like the contents of the file system or the
 /// response of some remote server.
-#[allow(clippy::cognitive_complexity)]
 pub fn get_config<E: EthSpec>(
     cli_args: &ArgMatches,
     spec_constants: &str,