code: create util.allocator and use globally

river is not a library and passing a general purpose allocators around
everywhere does not make sense and leads to ugly code. This does not
prevent us from using local arenas if they are fitting.

river/Root.zig

@@ -95,7 +95,7 @@ pub fn deinit(self: *Self) void {
 
 pub fn addOutput(self: *Self, wlr_output: *c.wlr_output) void {
     // TODO: Handle failure
-    const node = self.outputs.allocateNode(self.server.allocator) catch unreachable;
+    const node = self.outputs.allocateNode(util.allocator) catch unreachable;
     node.data.init(self, wlr_output) catch unreachable;
     self.outputs.append(node);