added read/writeFile helpers, updated tests to use new FS APIs

 - added readFile / writeFile functions, the goal here being to replace and add a complementary function to createDataFile
 - updated `test_fs_base` and `test_unistd_*` to use new FS APIs
 - started work on FS docs locally, updated API such that non-public facing functions were prefixed with underscores. this is kind of ugly, but I suppose it let's us not feel bad about breaking code where people have depended on private functionality
 - final commit just re-arranges blocks of functionality in library_fs to match up with how it's grouped in the docs

ran o1 and other, best to review the commits separately