Refactor code structure

Move X.509 and Openpgp operations into custom packages. Implement more
robust input reading. Do not convert []byte to string unnecessarily.

Finish implementation of X.509 CRL creation.

signer/common/helpers.go

@@ -0,0 +1,17 @@
+package common
+
+import (
+	"fmt"
+	"math/big"
+	"strconv"
+	"strings"
+)
+
+func StringAsBigInt(data []byte) (*big.Int, error) {
+	dataString := strings.TrimSpace(string(data))
+	parseInt, err := strconv.ParseInt(dataString, 16, 64)
+	if err != nil {
+		return nil, fmt.Errorf("could not parse %s as big int: %v", dataString, err)
+	}
+	return big.NewInt(parseInt), nil
+}