Proposal #3339: Add (+>) as a synonym for mappend

[Yitzchak Gale]

Thomas Schilling wrote:
> My argument would be that since Monoid is used more commonly,
> it should get the nice and short name (<>).

If it is defined in Data.Semigroup, then it can be used for
both.

The only reason Monoid is currently so much more common is
because the semigroups package is fairly new. Semigroups are
simple and quite ubiquitous. We should be strongly encouraging
their use, not discouraging it by creating an awkward namespace
clash.

Thanks,
Yitz