Implement "roles" into GHC.
Roles are a solution to the GeneralizedNewtypeDeriving type-safety problem. Roles were first described in the "Generative type abstraction" paper, by Stephanie Weirich, Dimitrios Vytiniotis, Simon PJ, and Steve Zdancewic. The implementation is a little different than that paper. For a quick primer, check out Note [Roles] in Coercion. Also see http://ghc.haskell.org/trac/ghc/wiki/Roles and http://ghc.haskell.org/trac/ghc/wiki/RolesImplementation For a more formal treatment, check out docs/core-spec/core-spec.pdf. This fixes Trac #1496, #4846, #7148.
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