File metadata API with symlink-awareness

There is a need for a more general file [`Metadata`](https://doc.rust-lang.org/std/fs/struct.Metadata.html) API with better awareness of symbolic links:

* Lots of operations today traverse symbolic links, which is not always what users want.
* Users may be interested in collecting a snapshot of all the metadata at once (akin to [stat(2)](https://man7.org/linux/man-pages/man2/lstat.2.html)), which is more efficient than doing it multiple times and avoids non-atomicity.

There is already a sketch of this API as an internal data type: https://github.com/haskell/directory/blob/master/System/Directory/Internal/Posix.hsc#L150

What needs to be done next is to stabilize this into a usable public API.