makes crds fields private (#13703)

Crds fields should maintain several invariants between themselves, so
exposing them as public fields can be bug prone. In addition these
invariants are asserted on every write:
https://github.com/solana-labs/solana/blob/9668dd85d/core/src/crds.rs#L138-L154
https://github.com/solana-labs/solana/blob/9668dd85d/core/src/crds.rs#L239-L262
which adds extra instructions and is not optimal. Should these fields be
private the asserts will be redundant.

core/src/crds_shards.rs

@@ -21,13 +21,11 @@ impl CrdsShards {
         }
     }
 
-    #[must_use]
     pub fn insert(&mut self, index: usize, value: &VersionedCrdsValue) -> bool {
         let hash = CrdsFilter::hash_as_u64(&value.value_hash);
         self.shard_mut(hash).insert(index, hash).is_none()
     }
 
-    #[must_use]
     pub fn remove(&mut self, index: usize, value: &VersionedCrdsValue) -> bool {
         let hash = CrdsFilter::hash_as_u64(&value.value_hash);
         self.shard_mut(hash).swap_remove(&index).is_some()