add a type for the key phase and use it for header parsing and writing

internal/protocol/protocol.go

@@ -34,6 +34,27 @@ func (t PacketType) String() string {
 	}
 }
 
+// KeyPhase is the key phase
+type KeyPhase bool
+
+const (
+	// KeyPhaseZero is key phase 0
+	KeyPhaseZero KeyPhase = false
+	// KeyPhaseOne is key phase 1
+	KeyPhaseOne KeyPhase = true
+)
+
+func (p KeyPhase) String() string {
+	if p == KeyPhaseZero {
+		return "0"
+	}
+	return "1"
+}
+
+func (p KeyPhase) Next() KeyPhase {
+	return !p
+}
+
 // A ByteCount in QUIC
 type ByteCount uint64