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_config.h

@@ -34,4 +34,12 @@ static inline lfs_word_t lfs_min(lfs_word_t a, lfs_word_t b) {
     return (a < b) ? a : b;
 }
 
+static inline lfs_word_t lfs_ctz(lfs_word_t a) {
+    return __builtin_ctz(a);
+}
+
+static inline lfs_word_t lfs_npw2(lfs_word_t a) {
+    return 32 - __builtin_clz(a-1);
+}
+
 #endif