implement a string representation of the version number

This will be useful as soon as we start supporting non-gQUIC versions.

internal/protocol/version.go

@@ -1,5 +1,7 @@
 package protocol
 
+import "fmt"
+
 // VersionNumber is a version number as int
 type VersionNumber int
 
@@ -27,6 +29,21 @@ func (vn VersionNumber) UsesTLS() bool {
 	return vn == VersionTLS
 }
 
+func (vn VersionNumber) String() string {
+	switch vn {
+	case VersionWhatever:
+		return "whatever"
+	case VersionUnsupported:
+		return "unsupported"
+	case VersionUnknown:
+		return "unknown"
+	case VersionTLS:
+		return "TLS dev version (WIP)"
+	default:
+		return fmt.Sprintf("%d", vn)
+	}
+}
+
 // VersionNumberToTag maps version numbers ('32') to tags ('Q032')
 func VersionNumberToTag(vn VersionNumber) uint32 {
 	v := uint32(vn)