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

[Yitzchak Gale]

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