WIP: Disable CUSKs by default

Vladislav Zavialovrequested to merge
wip/testsuite-no-cusks into master

This patch updates deprecates the -XCUSKs extension, turns it off by default, and updates the libraries and the testsuite to use standalone kind signatures instead.

According to GHC Proposal #54, we should have done so by 8.8, but we're obviously behind schedule.

Edited by Vladislav Zavialov

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