Inconsistent use of wrapped arguments

[john-romein]

Some members of the Stream class take DeviceMemory as an argument, others a CUdeviceptr. This should be consistent. In general, I think we should use wrapped objects as arguments, as this is the common case. And why is the DeviceMemory(CUdeviceptr) constructor explicit now? This prevents the user from passing a CUdeviceptr as an actual argument, which would otherwise have been converted implicitly to a DeviceMemory argument.