Fixes#681.
Relying on memory allocated by `std::vector` via direct pointers is dangerous,
as vectors may move buffers during resize. This fix puts the `DeviceScan`
allocation into a `str::shared_ptr`, which 1) allocates the memory on heap and
2) keeps track of the memory through reference counting. Thus it is safe to
store the `shared_ptr` across different vectors/maps.
