This change is to prevent needed a separate stat syscall for each file
when using readdir.
For consistency, this PR also modifies std's `WalkEntry` interface to
extend `DirEntry` with an additional `path` field.
* My original implementation of `fs.appendFile` used an async API, which, though
it would work fine as a polyfill, wasn't an exact match with the Node API. This PR
reworks that API to mimic the Node API fully as a synchronous void function with
an async internal implementation.
* Refactor move of other internal fs `dirent` and `dir` classes to the _fs internal
directory.
