* `std.fs.Dir.Entry.Kind` is moved to `std.fs.File.Kind`
* `std.fs.File.Stat` gains the `kind` field, so performing a stat() on
a File now tells what kind of file it is. On Windows this only will
distinguish between directories and files.
* rework zig fmt logic so that in the case of opening a file and
discovering it to be a directory, it closes the file descriptor
before re-opening it with O_DIRECTORY, using fewer simultaneous open
file descriptors when walking a directory tree.
* rework zig fmt logic so that it pays attention to the kind of
directory entries, and when it sees a sub-directory it attempts to
open it as a directory rather than a file, reducing the number of
open() syscalls when walking a directory tree.
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