Deny warnings on CI (#1372)

## Issue Addressed

Prevent CI from succeeding when there are warnings. Code can still be built and tested with warnings locally, but CI may fail during a Rust update (which is fine IMO).

## Proposed Changes

* Deny warnings for all stable compiler jobs on CI (excludes `cargo udeps`, which runs under nightly)
* Fix the warnings currently on `master` related to unnecessary `mem::replace`

consensus/types/src/beacon_state/tree_hash_cache.rs

@@ -211,7 +211,7 @@ impl ValidatorsListTreeHashCache {
             .list_cache
             .recalculate_merkle_root(&mut list_arena, leaves.into_iter())?;
 
-        std::mem::replace(&mut self.list_arena, list_arena);
+        self.list_arena = list_arena;
 
         Ok(mix_in_length(&list_root, validators.len()))
     }