core/types: encode receipt status in PostState field

This fixes a regression where the new Failed field in ReceiptForStorage
rejected previously stored receipts. Fix it by removing the new field
and store status in the PostState field. This also removes massive RLP
hackery around the status field.

core/database_util_test.go

@@ -340,7 +340,7 @@ func TestBlockReceiptStorage(t *testing.T) {
 	db, _ := ethdb.NewMemDatabase()
 
 	receipt1 := &types.Receipt{
-		PostState:         []byte{0x01},
+		Failed:            true,
 		CumulativeGasUsed: big.NewInt(1),
 		Logs: []*types.Log{
 			{Address: common.BytesToAddress([]byte{0x11})},
@@ -351,7 +351,7 @@ func TestBlockReceiptStorage(t *testing.T) {
 		GasUsed:         big.NewInt(111111),
 	}
 	receipt2 := &types.Receipt{
-		PostState:         []byte{0x02},
+		PostState:         common.Hash{2}.Bytes(),
 		CumulativeGasUsed: big.NewInt(2),
 		Logs: []*types.Log{
 			{Address: common.BytesToAddress([]byte{0x22})},