Removed a layer of indirection for index-list lookup

Files are now stored directly in the index-list, instead of being
referenced by pointers that used to live there. This somewhat reduces
the complexity around handling files, while still keeping the O(logn)
lookup cost.

lfs.h

@@ -125,13 +125,16 @@ typedef struct lfs_file {
     lfs_block_t head;
     lfs_size_t size;
 
+    lfs_off_t wpos;
     lfs_block_t wblock;
-    uint32_t windex;
+    lfs_off_t woff;
 
+    lfs_off_t rpos;
     lfs_block_t rblock;
-    uint32_t rindex;
     lfs_off_t roff;
 
+    int flags;
+
     struct lfs_entry entry;
 } lfs_file_t;