hash table should include a distinguishable "default" entry

[noel2004]

There is a default entry in hash table: (0, 0) -> 0, which is required for unassigned rows. However, this entry would induce problem (i.e. we would also lookup (0, 0) for a zero byte32 in StorevalueGadge)

We should turn the lookup table for hash into 4 rolws [<enable>, <input1>, <input2>, <hash>]