message: update implementation

Add more of the Message class (and various helpers), which are to replace all
the `mu-msg-*` code.

Add more tests.

lib/message/mu-message-part.hh

@@ -0,0 +1,101 @@
+/*
+** Copyright (C) 2022 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
+**
+** This program 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, or (at your option) any
+** later version.
+**
+** This program 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 this program; if not, write to the Free Software Foundation,
+** Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+**
+*/
+
+
+#ifndef MU_MESSAGE_PART_HH__
+#define MU_MESSAGE_PART_HH__
+
+#include <string>
+#include <memory>
+#include <utils/mu-option.hh>
+#include <utils/mu-result.hh>
+
+namespace Mu {
+
+struct MimeObject; // forward declaration
+
+class MessagePart {
+public:
+	/**
+	 * Construct MessagePart from a MimeObject
+	 *
+	 * @param obj
+	 */
+	MessagePart(const MimeObject& obj);
+
+	/**
+	 * Copy CTOR
+	 *
+	 * @param other
+	 */
+	MessagePart(const MessagePart& other);
+
+	/**
+	 * DTOR
+	 *
+	 */
+	~MessagePart();
+
+	/**
+	 * Filename for the mime-part
+	 *
+	 * @return the filename or Nothing if there is none
+	 */
+	Option<std::string> filename() const noexcept;
+
+	/**
+	 * Mime-type for the mime-part (e.g. "text/plain")
+	 *
+	 * @return the mime-part or Nothing if there is none
+	 */
+	Option<std::string> mime_type() const noexcept;
+
+	/**
+	 * Get the length of the (unencoded) MIME-part.
+	 *
+	 * @return the size
+	 */
+	size_t size() const noexcept;
+
+	/**
+	 * Write (decoded) mime-part contents to string
+	 *
+	 * @return a string or nothing if there is no contemt
+	 */
+	Option<std::string> to_string() const noexcept;
+
+
+	/**
+	 * Write (decoded) mime part to a file
+	 *
+	 * @param path path to file
+	 * @param overwrite whether to possibly overwrite
+	 *
+	 * @return size of file or or an error.
+	 */
+	Result<size_t> to_file(const std::string& path, bool overwrite) const noexcept;
+
+	struct Private;
+private:
+	std::unique_ptr<MimeObject> mime_obj;
+};
+
+} // namespace Mu
+
+#endif /* MU_MESSAGE_PART_HH__ */