fold, foldMap, foldl, & foldr

[robrix]

This PR introduces tfold, tfoldMap, tfoldl, & tfoldr operations, by analogy with Foldable’s fold, foldMap, foldl, & foldr, but (in the case of tfold & tfoldMap) using a Category instance instead of a Monoid instance to combine elements.

As a fun aside, since all the various type-aligned sequence types have Category instances, we can use this to reconstruct one kind of sequence with another by using tfoldMap tsingleton. We can also do tfoldr (<|) tempty and tfoldl (|>) tempty, which is quite satisfying.

I think this resolves #4, tho I’d note that the types I arrived at differ from the ones mentioned here: https://stackoverflow.com/questions/30985070/how-can-i-express-foldr-in-terms-of-foldmap-for-type-aligned-sequences — in particular, the types mentioned there don’t admit the typical recursive definitions of left & right folds for ConsList, whereas the ones I’ve used do; and intuitively, tfoldr’s z parameter is its terminal value, whereas tfoldl’s z is its initial value.

I attempted to follow base’s definition of foldl with a Dual newtype, but I found that difficult; I think we would need Dual (Endo (Dual h)) rather than Dual (Endo h), tho I’m not at all certain of that. Nevertheless, I’m pretty satisfied with the result.

Thank you for your time & consideration!

[treeowl]

While you're waiting for the maintainer to respond, perhaps you should add tfoldl' and tfoldr'.

[treeowl]

I would also recommend extracting all this into a separate class, and making it a superclass of TASequence, despite the potential compatibility issues. It's also possible to write an analogue of Traversable.

[robrix]

I’m afraid I gave up on this somewhere around the second year.