Fix new clippy lints (#2036)

## Issue Addressed

NA

## Proposed Changes

Fixes new clippy lints in the whole project (mainly [manual_strip](https://rust-lang.github.io/rust-clippy/master/index.html#manual_strip) and [unnecessary_lazy_evaluations](https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_lazy_evaluations)). Furthermore, removes `to_string()` calls on literals when used with the `?`-operator.

consensus/types/src/beacon_state.rs

@@ -884,7 +884,7 @@ impl<T: EthSpec> BeaconState<T> {
         self.eth1_data
             .deposit_count
             .checked_sub(self.eth1_deposit_index)
-            .ok_or_else(|| Error::InvalidDepositState {
+            .ok_or(Error::InvalidDepositState {
                 deposit_count: self.eth1_data.deposit_count,
                 deposit_index: self.eth1_deposit_index,
             })

consensus/types/src/beacon_state/committee_cache.rs

@@ -56,7 +56,7 @@ impl CommitteeCache {
             &seed[..],
             false,
         )
-        .ok_or_else(|| Error::UnableToShuffle)?;
+        .ok_or(Error::UnableToShuffle)?;
 
         // The use of `NonZeroUsize` reduces the maximum number of possible validators by one.
         if state.validators.len() == usize::max_value() {
@@ -148,7 +148,7 @@ impl CommitteeCache {
     pub fn get_all_beacon_committees(&self) -> Result<Vec<BeaconCommittee>, Error> {
         let initialized_epoch = self
             .initialized_epoch
-            .ok_or_else(|| Error::CommitteeCacheUninitialized(None))?;
+            .ok_or(Error::CommitteeCacheUninitialized(None))?;
 
         initialized_epoch.slot_iter(self.slots_per_epoch).try_fold(
             Vec::with_capacity(self.slots_per_epoch as usize),

consensus/types/src/beacon_state/tree_hash_cache.rs

@@ -419,7 +419,7 @@ impl ParallelValidatorTreeHash {
 
                         let validator = validators
                             .get(val_index)
-                            .ok_or_else(|| Error::TreeHashCacheInconsistent)?;
+                            .ok_or(Error::TreeHashCacheInconsistent)?;
 
                         validator
                             .recalculate_tree_hash_root(arena, cache)