p2p: move rlpx into separate package (#21464)

This change moves the RLPx protocol implementation into a separate package,
p2p/rlpx. The new package can be used to establish RLPx connections for
protocol testing purposes.

Co-authored-by: Felix Lange <fjl@twurst.com>

p2p/message_test.go

@@ -18,11 +18,9 @@ package p2p
 
 import (
 	"bytes"
-	"encoding/hex"
 	"fmt"
 	"io"
 	"runtime"
-	"strings"
 	"testing"
 	"time"
 )
@@ -141,12 +139,3 @@ func TestEOFSignal(t *testing.T) {
 	default:
 	}
 }
-
-func unhex(str string) []byte {
-	r := strings.NewReplacer("\t", "", " ", "", "\n", "")
-	b, err := hex.DecodeString(r.Replace(str))
-	if err != nil {
-		panic(fmt.Sprintf("invalid hex string: %q", str))
-	}
-	return b
-}