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/tree_hash/src/impls.rs

@@ -22,7 +22,7 @@ macro_rules! impl_for_bitsize {
                 HASHSIZE / ($bit_size / 8)
             }
 
-            #[allow(clippy::cast_lossless)]
+            #[allow(clippy::cast_lossless)] // Lint does not apply to all uses of this macro.
             fn tree_hash_root(&self) -> Hash256 {
                 int_to_hash256(*self as u64)
             }