Iteration optimisation #107
                
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This is built on top of #104 and #96, and trussed-dev/littlefs2#33
This PR:
Uses the apis added to trussed-dev/littlefs2#33 to improve the skipping that happens each time in$O(n)$  (with n being the number of files in the directory, but it should be much faster.
read_dir(_files)_next(). I think the operation is likely stillIt also exposes$O(n^2)$  operation
read_dir(_files)_nthsyscalls that make it possible to resume iteration at a certain point without having to iterate n time withread_dir(_files)_next, which is a