Introduce MemTag. Use it in SparseIndexList. Adapt codegen to use entities and views.

 - Introduce the MemTag protocol (from my master's thesis)
 - Implement CUDA and HIP unified memory allocator
 - Adapt SparseIndexList to use memtags and select allocators according to them
 - Adapt codegen to use the entity/view pattern for all fields such that the block data ID of the SparseIndexList is now fully obscured
 - Make FreeSlipPipe test case run on GPU