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.