This patch finally adds 'left' and 'right' coercions back into
GHC.  Trac #7205 gives the details.

The main change is to add a new constructor to Coercion:

  data Coercion
    = ...
    | NthCo  Int         Coercion     -- OLD, still there
    | LRCo   LeftOrRight Coercion     -- NEW

  data LeftOrRight = CLeft | CRight

Plus:
  * Similar change to TcCoercion
  * Use LRCo when decomposing AppTys
  * Coercion optimisation needs to handle left/right

The rest is just knock-on effects.