Allow Haddock markup in deprecation messages

[SimonHengel]

It is very common to refer to an other function/type in the message associated with a DEPRECATED pragma.

Here is an example.

{-# DEPRECATED foo "use `bar` instead` #-}

We already include the deprecation message in the documentation for foo, but it would also be nice to create a hyperlink to bar.

[anonymous]

Milestone 2.10.0 deleted