fs-mistrust: Add read_directory and remove_file to CheckedDir.

Nick Mathewsonrequested to merge
nickm/arti:checkeddir_list_and_remove into main

Closes #1117 (closed).

@gabi-250, does this do what you had in mind?

To the reviewer: Please think hard about whether I have screwed up any subtle filesystem permissions errors here in the fs-mistrust model.

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