Muzzle clippy type complexity warning.

Despite clippy's suggestion, it is not currently possible to create type aliases
for traits and so everything within the 'Box<...>' cannot be type aliased.

This then leaves creating full blown traits, and either implementing
said traits by closure (somehow) or moving the closures into new structs
implementing said traits which seems a bit of a palaver.

Alternatively it is possible to define and use the type alias 'type ResultBox<T> = Result<Box<T>>'
which does seems rather pointless and not a great reduction in complexity but is enough to keep clippy happy.

In the end I simply went with squelching the clippy warning.

ledger/src/snapshot_utils.rs

@@ -418,6 +418,7 @@ where
     Ok(ret)
 }
 
+#[allow(clippy::type_complexity)]
 fn create_versioned_bank_snapshot_serializer<'a>(
     snapshot_version: SnapshotVersion,
     bank: &'a Bank,
@@ -451,6 +452,7 @@ fn create_versioned_bank_snapshot_serializer<'a>(
     }
 }
 
+#[allow(clippy::type_complexity)]
 fn create_versioned_bank_snapshot_deserializer<'a, P: AsRef<Path>>(
     snapshot_version: &str,
     account_paths: &'a [PathBuf],