Proposal: add Monoid1 and Semigroup1 classes

[David Feuer]

I've been playing around with the idea of writing Haskell 2010 type classes
for finite sequences and non-empty sequences, somewhat similar to Michael
Snoyman's Sequence class in mono-traversable. These are naturally based on
Monoid1 and Semigroup1, which I think belong in base.

class Semigroup1 f where
  (<<>>) :: f a -> f a -> f a
class Semigroup1 f => Monoid1 f where
  mempty1 :: f a

Then I can write

class (Monoid1 t, Traversable t) => Sequence t where
  singleton :: a -> t a
  -- and other less-critical methods

class (Semigroup1 t, Traversable1 t) => NESequence where
  singleton1 :: a -> t a
  -- etc.

I can, of course, just write my own, but I don't think I'm the only one
using such.
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