Remove "list" from rvar class definition.

[mjskay]

Summary

This PR closes #269 and #267 by removing "list" from the end of the class definition of rvar. This ensures that vctrs::vec_is_list(<rvar>) is FALSE (fixing some issues with dplyr, closing #269) and ensures that the S4 class definition of "rvar" does not redefine the S4 parent class of "vctrs_vctr" to be "list" (closing #267).

Since we already discussed this on #269, once the tests pass here I will merge.

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• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)