Remove the additional layer of Coercible over EqR

And instead create Coercible direclty as a class with what was EqR as
the TyCon.
Prevent manual instances of Coercible
Mark constructors used in Coercible deriving as used

(to avoid spurious warnigs with -fwarn-unused-imports)

The implementation breaks the TcS abstraction a bit and can be
possibly be refactored.
Coercible: Do not check for data constructors

as per
http://www.haskell.org/pipermail/ghc-devs/2013-September/002327.html
Typo