Add combinators on Semigroup, Monoid, Aggregator to reverse order.

[johnynek]

see comment here:

#739 (comment)

reversing the order of semigroup preserves it being a semigroup, as does putting a constant element between every pair. This could be a combinator on semigroup that we could propagate up to other Semigroup related abstractions, e.g. Aggregator, Fold, Scan.

[regadas]

Hi @johnynek just saw your change @ typelevel/cats#3279 ... I think it will be handy for this one.