Add "shortcut" function for getting the default device

[eyalroz]

This:

cuda::device::get(cuda::device::default_device_id);

is too verbose of a command for getting the default device. Instead, let's make it:

cuda::device::default_();

(I was mulling over the choice of function name, between this and get_default(), and went with the shorter one; comment if you disagree.)