Factor out device, event, stream etc. identification code

[eyalroz]

We currently do a lot of string generation of the form

:std::string("stream ") + cuda::detail_::ptr_as_hex(handle) + " on device " + std::to_string(device_id);

and this is quite redundant. Let us, instead, have identify() functions, with variants depending on the information we have about these objects, within the detail_ namespace. We already have this, albeit imperfectly, on the driver-wrappers branch.