Remove SyncRequest from K::Object and create a new K::Session type

This is a first step at fixing the conceptual insanity that is our
handling of service and IPC calls. For now, interfaces still directly
derived from Session because we don't have the infrastructure to do it
properly. (That is, Processes and scheduling them.)

src/core/hle/service/hid_user.cpp

@@ -153,7 +153,7 @@ void PadUpdateComplete() {
  *      8 : Event signaled by HID_User
  */
 static void GetIPCHandles(Service::Interface* self) {
-    u32* cmd_buff = Service::GetCommandBuffer();
+    u32* cmd_buff = Kernel::GetCommandBuffer();
 
     cmd_buff[1] = 0; // No error
     cmd_buff[3] = shared_mem;