[Haskell-cafe] Please add a method for optimized concat to the Semigroup class

Hi Edward, Thanks much for the very useful semigroups package. When using it in practice, it would be very useful to have an analogue to the mconcat method of Monoid. It has the obvious default implementation, but allows for an optimized implementation for specific instances. That turns out to be something that comes up all the time (at least for me) in real life. Thanks, Yitz