Reader and writer for a simple CUTable format

[vuule]

Description

Adds reader and writer for a format based on cudf::pack and cudf::unpack. The format does not encode or compress data. The reader is zero-copy when the packed data is already on device, so the returned type is not cudf::table. Instead, a packed_table object that owns the device memory and exposes a cudf::table_view is returned.

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [ ] The documentation is up to date with these changes.

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