Replace SongList with struct songvec

Our linked-list implementation is wasteful and the
SongList isn't modified enough to benefit from being a linked
list.  So use a more compact array of song pointers which
saves ~200K on a library with ~9K songs (on x86-32).

src/directory.h

@@ -20,13 +20,14 @@
 #define DIRECTORY_H
 
 #include "song.h"
+#include "songvec.h"
 
 typedef List DirectoryList;
 
 typedef struct _Directory {
 	char *path;
 	DirectoryList *subDirectories;
-	SongList *songs;
+	struct songvec songs;
 	struct _Directory *parent;
 	ino_t inode;
 	dev_t device;