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These are for working with the filesystem when using pyboard.py as a
library, rather than at the command line.

- fs_listdir returns a list of tuples, in the same format as os.ilistdir().
- fs_readfile returns the contents of a file as a bytes object.
- fs_writefile allows writing a bytes object to a file.
- fs_stat returns an os.statresult.

All raise FileNotFoundError (or OSError(ENOENT) on Python 2) if the file is
not found (or PyboardError on other errors).

Updated fs_cp and fs_get to use fs_stat to compute file size.

This work was funded through GitHub Sponsors.
Signed-off-by: Jim Mussared <jim.mussared@gmail.com>