vkr: add helpers to manipulate the object table

This adds vkr_device_{add,remove}_object for device objects and vkr_context_{add,remove}_object for non-device objects.

This MR only identifies whether an object is a device object or not, and picks the appropriate helper. Based on this, I plan to

• make sure we don't hit the asserts (id is non-zero and unused) in vkr_context_add_object
• automatically add device objects to dev->objects in vkr_device_add_object
• optionally and unlikely, add a complete set of helpers
  • vkr_context_add_instance
  • vkr_instance_add_physical_device
  • vkr_physical_device_add_device
  • vkr_device_add_queue
  • vkr_descriptor_pool_add_descriptor_set
  • vkr_command_pool_add_command_buffer

/cc @zzyiwei @ryanneph