DetachedSong: fork of struct Song

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.

src/SongUpdate.cxx

@@ -78,8 +78,6 @@ tag_scan_fallback(Path path,
 bool
 Song::UpdateFile()
 {
-	assert(IsFile());
-
 	const auto path_fs = map_song_fs(*this);
 	if (path_fs.IsNull())
 		return false;
@@ -107,13 +105,9 @@ Song::UpdateFile()
 bool
 Song::UpdateFileInArchive()
 {
-	const char *suffix;
-
-	assert(IsFile());
-
 	/* check if there's a suffix and a plugin */
 
-	suffix = uri_get_suffix(uri);
+	const char *suffix = uri_get_suffix(uri);
 	if (suffix == nullptr)
 		return false;