Adopted ctz skip-list structure earlier than expected

The primary data structure backing the little fs was planned
to be a little ctz based skip-list for O(logn) lookup and
O(1) append.

Was initially planning to start with a simple linked list of
index blocks, but was having trouble implementing the free-list
on top of the structure. Went ahead and adopted the skip-list
structure since it may have actually been easier.

lfs.h

@@ -11,11 +11,10 @@
 #include "lfs_bd.h"
 
 struct lfs_free_list {
-    lfs_word_t rev[2];
-    lfs_ino_t phead;
     lfs_ino_t head;
-    lfs_ino_t tip;
-    lfs_off_t off;
+    lfs_word_t ioff;
+    lfs_word_t icount;
+    lfs_word_t rev;
 };
 
 typedef struct lfs {