Fix utils.hash_function typing

tests/core/pyspec/eth2spec/utils/hash_function.py

@@ -1,28 +1,17 @@
 from hashlib import sha256
+from typing import Dict, Union
 
 ZERO_BYTES32 = b'\x00' * 32
 
 
-def _hash(x):
+def _hash(x: Union[bytes, bytearray, memoryview]) -> bytes:
     return sha256(x).digest()
 
 
-# Minimal collection of (key, value) pairs, for fast hash-retrieval, to save on repetitive computation cost.
-# Key = the hash input
-# Value = the hash output
-hash_cache = []
+hash_cache: Dict[bytes, bytes] = {}
 
 
-def add_zero_hashes_to_cache():
-    zerohashes = [(None, ZERO_BYTES32)]
-    for layer in range(1, 32):
-        k = zerohashes[layer - 1][1] + zerohashes[layer - 1][1]
-        zerohashes.append((k, _hash(k)))
-    hash_cache.extend(zerohashes[1:])
-
-
-def hash(x):
-    for (k, h) in hash_cache:
-        if x == k:
-            return h
+def hash(x: bytes) -> bytes:
+    if x in hash_cache:
+        return hash_cache[x]
     return _hash(x)