WIP: Use Semigroup's (<>) for Doc/SDoc

Before this patch, Outputable.hs defined its own (<>) which caused conflicts with (Data.Semigroup.<>) and thus led to inconvenience.

However, replacing it is not trivial due to a different fixity:

http://www.haskell.org/pipermail/libraries/2011-November/017066.html

Nevertheless, it is possible to update the pretty-printing code to work with (<>) of a different fixitiy, and that's what this patch implements.

Now Doc and SDoc are instances of Semigroup.