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/types/src/beacon_state/pubkey_cache.rs

@@ -23,11 +23,13 @@ impl PubkeyCache {
     ///
     /// The added index must equal the number of validators already added to the map. This ensures
     /// that an index is never skipped.
-    #[allow(clippy::integer_arithmetic)]
     pub fn insert(&mut self, pubkey: PublicKeyBytes, index: ValidatorIndex) -> bool {
         if index == self.len {
             self.map.insert(pubkey, index);
-            self.len += 1;
+            self.len = self
+                .len
+                .checked_add(1)
+                .expect("map length cannot exceed usize");
             true
         } else {
             false

consensus/types/src/chain_spec.rs

@@ -587,9 +587,13 @@ impl Default for YamlConfig {
     }
 }
 
+#[allow(clippy::integer_arithmetic)] // Arith cannot overflow or panic.
+fn milliseconds_to_seconds(millis: u64) -> u64 {
+    millis / 1000
+}
+
 /// Spec v0.12.1
 impl YamlConfig {
-    #[allow(clippy::integer_arithmetic)]
     pub fn from_spec<T: EthSpec>(spec: &ChainSpec) -> Self {
         Self {
             config_name: T::spec_name().to_string(),
@@ -611,7 +615,7 @@ impl YamlConfig {
             hysteresis_upward_multiplier: spec.hysteresis_upward_multiplier,
             proportional_slashing_multiplier: spec.proportional_slashing_multiplier,
             bls_withdrawal_prefix: spec.bls_withdrawal_prefix_byte,
-            seconds_per_slot: spec.milliseconds_per_slot / 1000,
+            seconds_per_slot: milliseconds_to_seconds(spec.milliseconds_per_slot),
             min_attestation_inclusion_delay: spec.min_attestation_inclusion_delay,
             min_seed_lookahead: spec.min_seed_lookahead.into(),
             max_seed_lookahead: spec.max_seed_lookahead.into(),

consensus/types/src/graffiti.rs

@@ -18,7 +18,7 @@ pub struct Graffiti(#[serde(with = "serde_graffiti")] pub [u8; GRAFFITI_BYTES_LE
 
 impl Graffiti {
     pub fn as_utf8_lossy(&self) -> String {
-        #[allow(clippy::invalid_regex)]
+        #[allow(clippy::invalid_regex)] // This is a false positive, this regex is valid.
         let re = Regex::new("\\p{C}").expect("graffiti regex is valid");
         String::from_utf8_lossy(&re.replace_all(&self.0[..], &b""[..])).to_string()
     }