Use size_type in device_uvector definitions when accepting/returning something of size_type

[wence-]

device_uvector advertises the type used for the size of the vector as using size_type = std::size_t. But most of the definitions just explicitly spell out std::size_t. We should change these to use size_type (so that things align, and it is easier to change should one want to). This would make the signatures more aligned with those of std::vector as well.

Originally posted by @wence- in #1317 (comment)