Improve iteration capability

[sosthene-nitrokey]

Iteration over a directory is not very efficient, as it can't be safely paused and resumed.
This leads to issues. For example in trussed, as a consequence the iteration happens at a speed of $O(n^2)$ because of this: https://github.com/trussed-dev/trussed/blob/main/src/store/filestore.rs#L335

The raw c API exposes a lfs_dir_tell, which littlefs can use to "store" the current iteration position and return to using lfs_dir_seek

[sosthene-nitrokey]

Trying to implement it lead to discovering a bug in littelfs: littlefs-project/littlefs#785