Move offset_handler construction into self

eth2/utils/ssz/src/cached_tree_hash/impls.rs

@@ -4,15 +4,15 @@ use crate::{ssz_encode, Encodable};
 impl CachedTreeHash for u64 {
     type Item = Self;
 
-    fn build_cache(&self) -> Result<TreeHashCache, Error> {
-        TreeHashCache::from_bytes(merkleize(ssz_encode(self)))
+    fn leaves_and_subtrees(&self) -> Vec<u8> {
+        merkleize(ssz_encode(self))
     }
 
     fn num_bytes(&self) -> usize {
         8
     }
 
-    fn offset_handler(&self, _initial_offset: usize) -> Result<OffsetHandler, Error> {
+    fn offsets(&self) -> Result<Vec<usize>, Error> {
         Err(Error::ShouldNotProduceOffsetHandler)
     }