Add PoolL2Tx verify signature needed methods

Implements the hash to be signed from PoolL2Tx, compatible with javascript version.
Implements the signature verification for a given PoolL2Tx.

common/utils.go

@@ -1,5 +1,11 @@
 package common
 
+import (
+	"math/big"
+
+	ethCommon "github.com/ethereum/go-ethereum/common"
+)
+
 // SwapEndianness swaps the order of the bytes in the slice.
 func SwapEndianness(b []byte) []byte {
 	o := make([]byte, len(b))
@@ -8,3 +14,8 @@ func SwapEndianness(b []byte) []byte {
 	}
 	return o
 }
+
+// EthAddrToBigInt returns a *big.Int from a given ethereum common.Address.
+func EthAddrToBigInt(a ethCommon.Address) *big.Int {
+	return new(big.Int).SetBytes(a.Bytes())
+}