Implement UnliftedDatatypes

Note: f8c6fce4 and !5440 (closed) wrongly refer to this ticket instead of #19623 (closed). If you got here by following the reference in the commit message, see #19623 (closed) instead.

GHC proposal #265 defines the UnliftedDatatypes extension.

This ticket tracks the implementation of said extension.

Edited May 24, 2021 by Vladislav Zavialov

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