hash: add hash_short macro, state_s: add prev and move

src/hash.h

@@ -30,6 +30,11 @@
 
 typedef u64 hkey_t;                               /* cannot use typedef for key_t */
 
+/**
+ * hash_short: return the value of a hash first 7 MSB.
+ */
+#define hash_short(hash)  ((hash) >> (64 - 8))
+
 /**
  * hentry_t: hashtable bucket.
  *