Tyr: implement an AS slot allocator

Tyr can allocate AS spaces and mark them as in use, but there is no code to reclaim them.

In practice, we will start returning -EBUSY in perpetuity as soon as we exhaust the number of AS slots available.

Please implement a slot allocator that can mark slots as free when appropriate. The allocator should also return the LRU free slot when a new slot is requested. Add an IGT test to make sure that this actually works too.

One can look into AGX as prior art for this task.