feat: make BLOCKHASH return only last blockhash (ethereum#196)

* make BLOCKHASH return only last blockhash

* fix

* small fix

core/vm/instructions.go

@@ -440,10 +440,10 @@ func opBlockhash(pc *uint64, interpreter *EVMInterpreter, scope *ScopeContext) (
 	}
 	var upper, lower uint64
 	upper = interpreter.evm.Context.BlockNumber.Uint64()
-	if upper < 257 {
+	if upper < 2 {
 		lower = 0
 	} else {
-		lower = upper - 256
+		lower = upper - 1
 	}
 	if num64 >= lower && num64 < upper {
 		num.SetBytes(interpreter.evm.Context.GetHash(num64).Bytes())

core/vm/runtime/runtime_test.go

@@ -337,10 +337,10 @@ func TestBlockhash(t *testing.T) {
 	if first.Uint64() != 999 {
 		t.Fatalf("second block should be 999, got %d (%x)", first, ret[32:64])
 	}
-	if last.Uint64() != 744 {
-		t.Fatalf("last block should be 744, got %d (%x)", last, ret[64:96])
+	if last.Uint64() != 0 {
+		t.Fatalf("last block should be 0, got %d (%x)", last, ret[64:96])
 	}
-	if exp, got := 255, chain.counter; exp != got {
+	if exp, got := 0, chain.counter; exp != got {
 		t.Errorf("suboptimal; too much chain iteration, expected %d, got %d", exp, got)
 	}
 }