Add DeviceRef::{has_unified_memory, recommended_max_working_set_size, max_tranfer_rate}

[burtonageo]

Adds the following APIs:

• https://developer.apple.com/documentation/metal/mtldevice/3229025-hasunifiedmemory
• https://developer.apple.com/documentation/metal/mtldevice/2369280-recommendedmaxworkingsetsize
• https://developer.apple.com/documentation/metal/mtldevice/3114007-maxtransferrate

[kvark]

Thank you! Please rebase.

[burtonageo]

Done (I think) - my git skills aren't that great