core: allow transfer amount to be bytes

VM can produce both big.Int and []byte because they
are converted to each other on demand.

pkg/core/blockchain.go

@@ -707,9 +707,13 @@ func (bc *Blockchain) storeBlock(block *block.Block) error {
 						continue
 					}
 					amount, ok := arr[3].Value().(*big.Int)
+					if !ok {
+						bs, ok := arr[3].Value().([]byte)
 						if !ok {
 							continue
 						}
+						amount = emit.BytesToInt(bs)
+					}
 					// TODO: #498
 					_, _, _, _ = op, from, to, amount
 				}