Add TxID calculation & New{Layer}Tx Type

Add TxID calculation & New{Layer}Tx Type

New{Layer}Tx methods that compute the `TxID` & `TxType` values from the
transaction values:
- NewL1Tx
- NewL2Tx
- NewPoolL2Tx

Add TxID Scanner & Valuer for database/sql

HistoryDB & L2DB & API packages tests will need to be addapted to the
TestTransaction generation once is done.

common/l2tx_test.go

@@ -0,0 +1,20 @@
+package common
+
+import (
+	"math/big"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNewL2Tx(t *testing.T) {
+	l2Tx := &L2Tx{
+		FromIdx: 87654,
+		ToIdx:   300,
+		Amount:  big.NewInt(4),
+		Nonce:   144,
+	}
+	l2Tx, err := NewL2Tx(l2Tx)
+	assert.Nil(t, err)
+	assert.Equal(t, "0x020000000156660000000090", l2Tx.TxID.String())
+}