new dir module

This is a lower-level interface than `std::fs::ReadDir`. Notable differences:

   * can be opened from a file descriptor (as returned by `openat`, perhaps
     before knowing if the path represents a file or directory). Uses
     `fdopendir` for this, available on all Unix platforms as of
     rust-lang/libc#1018.

   * implements `AsRawFd`, so it can be passed to `fstat`, `openat`, etc.

   * can be iterated through multiple times without closing and reopening the
     file descriptor. Each iteration rewinds when finished.

   * returns entries for `.` (current directory) and `..` (parent directory).

   * returns entries' names as a `CStr` (no allocation or conversion beyond
     whatever libc does).

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