WIP making good progress, found a solution for the cycle issue

Found solution for cycle issue for tail+branch solution, though it does
require creating an extra metadata-pair on relocate.

Need to get passing the advanced relocation tests, this is currently
failing. Also need to figure out why I added the assert on tail end, it
may be related.

lfs.h

@@ -314,6 +314,7 @@ typedef struct lfs_mdir {
     bool erased;
     bool first; // TODO come on
     bool split;
+    bool mustrelocate; // TODO not great either
     lfs_block_t tail[2];
     lfs_block_t branch[2];
 } lfs_mdir_t;