Add Fold1 pattern synonym & utilities

[Topsii]

This change extends the API of Control.Foldl.NonEmpty by some of the exports suggested in #209:

• Fold1_ pattern synonym
• purely
• purely_
• premap
• handles
• foldOver
• folded1

I created a naive benchmark that compares the execution time of

• NonEmpty.map succ
• variant that uses Fold
• variant that uses FoldM
• variant that uses the current definition data Fold1 a b = forall x. Fold1 !(a -> x) !(x -> a -> x) !(x -> b)
  imported qualified as Foldl1
• variant that uses the alternative definition data Fold1 a b = Fold1 (a -> Fold a b) from Fold1 data type definition & adding more folds/utilities to Control.Foldl.NonEmpty #209
  imported qualified as L1

Note that the y-axis uses a logarithmic scale.

In this benchmark the current definition of the Fold1 data type clearly performs worse than the alternative definition of Fold1.
The alternative definition of Fold1 performs similar to the Fold(M) variants.
Finally, the Fold* variants all perform worse than NonEmpty.map succ.

Personally, I favor changing the definition of the Fold1 data type and turning the current definition into a pattern synonym. However, in #209 you mentioned you believe

that for most functions the existing definition promotes the greatest code reuse.

Thus, I left the current definition unchanged and added the alternative definition as a pattern synonym.

[Gabriella439]

Thank you for doing this!