Get arbitrary check passing (2.0) (#2710)

## Issue Addressed

NA

## Proposed Changes

This is a wholesale rip-off of #2708, see that PR for more of a description.

I've made this PR since @realbigsean is offline and I can't merge his PR due to Github's frustrating `target-branch-check` bug. I also changed the branch to `unstable`, since I'm trying to minimize the diff between `merge-f2f`/`unstable`. I'll just rebase `merge-f2f` onto `unstable` after this PR merges.

When running `make lint` I noticed the following warning:

```
warning: patch for `fixed-hash` uses the features mechanism. default-features and features will not take effect because the patch dependency does not support this mechanism
```

So, I removed the `features` section from the patch.

## Additional Info

NA


Co-authored-by: realbigsean <seananderson33@gmail.com>

consensus/types/src/beacon_state/committee_cache.rs

@@ -327,7 +327,7 @@ pub fn get_active_validator_indices(validators: &[Validator], epoch: Epoch) -> V
 }
 
 #[cfg(feature = "arbitrary-fuzz")]
-impl arbitrary::Arbitrary for CommitteeCache {
+impl arbitrary::Arbitrary<'_> for CommitteeCache {
     fn arbitrary(_u: &mut arbitrary::Unstructured<'_>) -> arbitrary::Result<Self> {
         Ok(Self::default())
     }

consensus/types/src/beacon_state/exit_cache.rs

@@ -62,7 +62,7 @@ impl ExitCache {
 }
 
 #[cfg(feature = "arbitrary-fuzz")]
-impl arbitrary::Arbitrary for ExitCache {
+impl arbitrary::Arbitrary<'_> for ExitCache {
     fn arbitrary(_u: &mut arbitrary::Unstructured<'_>) -> arbitrary::Result<Self> {
         Ok(Self::default())
     }

consensus/types/src/beacon_state/pubkey_cache.rs

@@ -43,7 +43,7 @@ impl PubkeyCache {
 }
 
 #[cfg(feature = "arbitrary-fuzz")]
-impl arbitrary::Arbitrary for PubkeyCache {
+impl arbitrary::Arbitrary<'_> for PubkeyCache {
     fn arbitrary(_u: &mut arbitrary::Unstructured<'_>) -> arbitrary::Result<Self> {
         Ok(Self::default())
     }

consensus/types/src/beacon_state/tree_hash_cache.rs

@@ -589,7 +589,7 @@ impl OptionalTreeHashCacheInner {
 }
 
 #[cfg(feature = "arbitrary-fuzz")]
-impl<T: EthSpec> arbitrary::Arbitrary for BeaconTreeHashCache<T> {
+impl<T: EthSpec> arbitrary::Arbitrary<'_> for BeaconTreeHashCache<T> {
     fn arbitrary(_u: &mut arbitrary::Unstructured<'_>) -> arbitrary::Result<Self> {
         Ok(Self::default())
     }

consensus/types/src/pending_attestation.rs

@@ -20,7 +20,7 @@ pub struct PendingAttestation<T: EthSpec> {
 }
 
 #[cfg(feature = "arbitrary-fuzz")]
-impl<T: EthSpec> arbitrary::Arbitrary for PendingAttestation<T> {
+impl<T: EthSpec> arbitrary::Arbitrary<'_> for PendingAttestation<T> {
     fn arbitrary(u: &mut arbitrary::Unstructured<'_>) -> arbitrary::Result<Self> {
         Ok(Self {
             aggregation_bits: <BitList<T::MaxValidatorsPerCommittee>>::arbitrary(u)?,