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
 - added default modes to mkdir (0777) / mkdev (0666)
 - updated `test_fs_base` and `test_unistd_*` to use new FS APIs. also, moved JS setup from external --pre-js scripts to inline EM_ASM blocks.
 - 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 and sockets, best to review the commits separately