[FEA] Strings scatter function needed for cudf

[davidwendt]

Current function needs to be reworked for cudf scatter port. Here is the proposed signature as discussed with @trevorsm7 in slack discussion:

template <typename MapIterator>
std::unique_ptr<cudf::column> scatter( strings_column_view const& source,
                                       MapIterator scatter_map,
                                       strings_column_view const& target,
                                       rmm::mr::device_memory_resource* mr = rmm::mr::get_default_resource(),
                                       cudaStream_t stream = 0 );

Also, the null mask in the output column will be set by the caller. It will be initialized by copying the target's null mask before returning.

[davidwendt]

Looks like scatter will also need a scalar version. Propose this:

template<typename MapIterator>
std::unique_ptr<column> scatter(
      string_scalar const& source,
      MapIterator scatter_iter, size_type scatter_rows,
      string_column_view const& target,
      rmm::mr::device_memory_resource* mr = rmm::mr::get_default_resource(),
      cudaStream_t stream = 0);

[jrhemstad]

Perhaps instead of two separate APIs for scattering a column/scalar, just make the source an iterator?