core: Add code to read/write "varints"

Adapted from Google protobufs.  For several cases, we want to support
e.g. file sizes up to guint64, but paying the cost of 8 bytes for each
number is too high.

This will be used for static deltas and sizes metadata.

src/libostree/ostree-varint.h

@@ -0,0 +1,34 @@
+/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*-
+ *
+ * Copyright (C) 2013 Colin Walters <walters@verbum.org>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library 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
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ */
+
+#pragma once
+
+#include <gio/gio.h>
+
+G_BEGIN_DECLS
+
+guint64 _ostree_read_varuint64 (const guint8   *buf,
+                                gsize           buflen,
+                                gsize          *bytes_read);
+
+void _ostree_write_varuint64 (GString *buf, guint64 n);
+
+G_END_DECLS
+