[FEA]: Debugger pretty-printers: `cuda::stream`/`cuda::stream_ref`

[Jacobfaib]

Create a lldb and gdb pretty printer for cuda::stream/cuda::stream_ref.

The printer should live under libcudacxx/share/libcudacxx/<debugger>/stream.py for each debugger.

In addition, the pretty printers should be exhaustively tested, with tests living under libcudacxx/test/debugging/stream. See the existing tests in that directory for inspiration.