Add unit tests for C++ helper code in `kernels`

[lockshaw]

There exist a number of definitions in kernels that aren't part of the kernels themselves, but are used in them. An (inexhaustive and potentially outdated) list of their header files are below:

• accessor.h
• allocation.h
• array_shape.h
• datatype_dispatch.h
• legion_dim.h (and its interaction with ff_dim_t)
• perf_metrics.h
• per_device_op_state.h
• profiling.h

The code in these files has not been updated in a while, and may not have been fully implemented. Update this code (for example, dtgen instead of visitable), make sure it's implemented, determine what makes sense to test, and write the corresponding unit tests

[lockshaw]

@oOTigger Do you mind taking this over? It's been partially implemented in #992, but as that was created a while ago it may need some updating, polishing, etc.