Restructured directory code

After quite a bit of prototyping, settled on the following functions:
- lfs_dir_alloc  - create a new dir
- lfs_dir_fetch  - load and check a dir pair from disk
- lfs_dir_commit - save a dir pair to disk
- lfs_dir_shift  - shrink a dir pair to disk
- lfs_dir_append - add a dir entry, creating dirs if needed
- lfs_dir_remove - remove a dir entry, dropping dirs if needed

Additionally, followed through with a few other tweaks

tests/test_format.sh

@@ -11,12 +11,16 @@ TEST
 
 echo "--- Invalid superblocks ---"
 ln -f -s /dev/null blocks/0
+ln -f -s /dev/null blocks/1
 tests/test.py << TEST
     lfs_format(&lfs, &config) => LFS_ERROR_CORRUPT;
 TEST
-rm blocks/0
+rm blocks/0 blocks/1
 
 echo "--- Basic mounting ---"
+tests/test.py << TEST
+    lfs_format(&lfs, &config) => 0;
+TEST
 tests/test.py << TEST
     lfs_mount(&lfs, &config) => 0;
     lfs_unmount(&lfs) => 0;