refactor smtp sending

* [x] move package creation logic to `pmapi.SendMessageReq`
* [ ] write test of package creation logic
    * [x] internal
    * [x] plain
    * [x] external encrypted
    * [ ] signature ???
    * [x] attachments

pkg/pmapi/debug.go

@@ -0,0 +1,43 @@
+// Copyright (c) 2020 Proton Technologies AG
+//
+// This file is part of ProtonMail Bridge.
+//
+// ProtonMail Bridge is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// ProtonMail Bridge is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with ProtonMail Bridge.  If not, see <https://www.gnu.org/licenses/>.
+
+package pmapi
+
+import "unicode/utf8"
+
+func printBytes(body []byte) string {
+	if utf8.Valid(body) {
+		return string(body)
+	}
+	enc := []rune{}
+	for _, b := range body {
+		switch {
+		case b == 9:
+			enc = append(enc, rune('⟼'))
+		case b == 13:
+			enc = append(enc, rune('↵'))
+		case b < 32, b == 127:
+			enc = append(enc, '◡')
+		case b > 31 && b < 127, b == 10:
+			enc = append(enc, rune(b))
+		default:
+			enc = append(enc, 9728+rune(b))
+		}
+	}
+
+	return string(enc)
+}