hash: Replace struct with a primitive type for hash values.

Remove the struct hash_entry and define a new type for hash values.
Most likely we will only need one member in this struct. and instead
of just using unsinged int. let's name a new type for clarity.

proc-cache.c

@@ -44,7 +44,7 @@ struct header {
 };
 
 union hash {
-	unsigned int  index;
+	hash_t         index;
 	unsigned char sha1[20];
 };