code: update to zig master in prep for 0.7.0

This commit makes the minimal necessary changes to get things working,
there are further changes which can be made to take advantage of new
features.

river/view_stack.zig

@@ -140,7 +140,7 @@ pub fn ViewStack(comptime T: type) type {
         pub fn iter(
             start: ?*Node,
             dir: Direction,
-            context: var,
+            context: anytype,
             filter: fn (*View, @TypeOf(context)) bool,
         ) Iter(@TypeOf(context)) {
             return .{ .it = start, .dir = dir, .context = context, .filter = filter };