Guard CUSKs behind a language pragma

GHC Proposal #36 describes a transition plan away from CUSKs and to
top-level kind signatures:

1. Introduce a new extension, -XCUSKs, on by default, that detects CUSKs
   as they currently exist.
2. We turn off the -XCUSKs extension in a few releases and remove it
   sometime thereafter.

This patch implements phase 1 of this plan, introducing a new language
extension to control whether CUSKs are enabled. When top-level kind
signatures are implemented, we can transition to phase 2.