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This commit paves the way to a richer and more structured representation of GHC error messages, as per GHC proposal #306. More specifically 'Messages' from 'GHC.Types.Error' now gains an extra type parameter, that we instantiate to 'ErrDoc' for now. Later, this will allow us to replace ErrDoc with something more structure (for example messages coming from the parser, the typechecker etc).
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