normalize localparts with unicode nfc when parsing

both when parsing our configs, and for incoming on smtp or in messages.
so we properly compare things like é and e+accent as equal, and accept the
different encodings of that same address.

message/from.go

@@ -42,6 +42,10 @@ func From(elog *slog.Logger, strict bool, r io.ReaderAt) (raddr smtp.Address, en
 	if err != nil {
 		return raddr, nil, nil, fmt.Errorf("bad domain in from address: %v", err)
 	}
-	addr := smtp.Address{Localpart: smtp.Localpart(from[0].User), Domain: d}
+	lp, err := smtp.ParseLocalpart(from[0].User)
+	if err != nil {
+		return raddr, nil, nil, fmt.Errorf("parsing localpart in from address: %v", err)
+	}
+	addr := smtp.Address{Localpart: lp, Domain: d}
 	return addr, p.Envelope, textproto.MIMEHeader(header), nil
 }

message/part.go

@@ -104,7 +104,7 @@ type Envelope struct {
 // Address as used in From and To headers.
 type Address struct {
 	Name string // Free-form name for display in mail applications.
-	User string // Localpart.
+	User string // Localpart, encoded as string. Must be parsed before using as Localpart.
 	Host string // Domain in ASCII.
 }