std.fs: Implement cross-platform metadata API

Implements a cross-platform metadata API, aiming to reduce unnecessary Unix-dependence of the `std.fs` api. Presently, all OSes beside Windows are treated as Unix; this is likely the best way to treat things by default, instead of explicitly listing each Unix-like OS.

Platform-specific operations are not provided by `File.Metadata`, and instead are to be accessed from `File.Metadata.inner`.

Adds:

- File.setPermissions() : Sets permission of a file according to a `Permissions` struct (not available on WASI)

- File.Permissions : A cross-platform representation of file permissions
  - Permissions.readOnly() : Returns whether the file is read-only
  - Permissions.setReadOnly() : Sets whether the file is read-only
  - Permissions.unixSet() : Sets permissions for a class (UNIX-only)
  - Permissions.unixGet() : Checks a permission for a class (UNIX-only)
  - Permissions.unixNew() : Returns a new Permissions struct to represent the passed mode (UNIX-only)

- File.Metadata : A cross-platform representation of file metadata
  - Metadata.size() : Returns the size of a file
  - Metadata.permissions() : Returns a `Permissions` struct, representing permissions on the file
  - Metadata.kind() : Returns the `Kind` of the file
  - Metadata.accessed() : Returns the time the file was last accessed
  - Metadata.modified() : Returns the time the file was last modified
  - Metadata.created() : Returns the time the file was created (this is an optional, as the underlying filesystem, or OS may not support this)

Methods of `File.Metadata` are also available for the below, so I won't repeat myself
The below may be used for platform-specific functionality

- File.MetadataUnix : The internal implementation of `File.Metadata` on Unices

- File.MetadataLinux : The internal implementation of `File.Metadata` on Linux

- File.MetadataWindows : The implementation of `File.Metadata` on Windows

Author: ominitay

lib/std/fs.zig

@@ -2231,6 +2231,31 @@ pub const Dir = struct {
     }
 
     pub const ChownError = File.ChownError;
+
+    const Permissions = File.Permissions;
+    pub const SetPermissionsError = File.SetPermissionsError;
+
+    /// Sets permissions according to the provided `Permissions` struct.
+    /// This method is *NOT* available on WASI
+    pub fn setPermissions(self: Dir, permissions: Permissions) SetPermissionsError!void {
+        const file: File = .{
+            .handle = self.fd,
+            .capable_io_mode = .blocking,
+        };
+        try file.setPermissions(permissions);
+    }
+
+    const Metadata = File.Metadata;
+    pub const MetadataError = File.MetadataError;
+
+    /// Returns a `Metadata` struct, representing the permissions on the directory
+    pub fn metadata(self: Dir) MetadataError!Metadata {
+        const file: File = .{
+            .handle = self.fd,
+            .capable_io_mode = .blocking,
+        };
+        return try file.metadata();
+    }
 };
 
 /// Returns a handle to the current working directory. It is not opened with iteration capability.