[FEA] Use libcu++ from GitHub once it is available

[jrhemstad]

Is your feature request related to a problem? Please describe.

libcu++ is available in the CUDA toolkit as of 10.2. Since we need to support 10.0, we've been required to depend on the "freestanding" library, which is the library precursor to libcu++, but it is not the same as libcu++.

Describe the solution you'd like

Currently, libcu++ proper is not available on GitHub. Once it is, we should drop the "freestanding" library and depend directly on libcu++ from GitHub.

Additional Context

Per #5359 (comment), we should be able to simplify libcudf by removing the custom timestamp/duration types and just using a type alias to the cuda::std::chrono types.

[jrhemstad]

closed by #6275