mu-scm: add full-message support (body/header)

Implement support for "header" and "body" procedures, with require loading the
message file from disk, and create a foreign object for the message.

We keep those alive in a vector, and hook up a finalizer.

Update docs & tests as well.

scm/mu-scm-types.hh

@@ -0,0 +1,55 @@
+/*
+** Copyright (C) 2025 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_SCM_TYPES_HH
+#define MU_SCM_TYPES_HH
+
+#include "lib/mu-store.hh"
+#include "message/mu-contact.hh"
+
+#include "mu-scm.hh"
+
+namespace Mu::Scm {
+
+/**
+ * Initialize SCM/Store support.
+ *
+ * @param store a store
+ */
+void init_store(const Mu::Store& store);
+
+/**
+ * Initialize SCM/Message support.
+ *
+ * @param store a store
+ */
+void init_message();
+
+/**
+ * Convert a Contact to an SCM
+ *
+ * @param contact a contact
+ *
+ * @return SCM
+ */
+SCM to_scm(const Contact& contact);
+
+} // Mu::Scm
+
+#endif /*MU_SCM_TYPES_HH*/