Add `Data.Sequence.toList`

[sjshuck]

It's annoying to have to import Data.Foldable just for this. It's the only module from containers with this issue, making the rationale strange:

containers/containers/docs/sequence.rst

Lines 207 to 210 in 0f3e38b

	There is no ``toList`` function in the Sequence module since it can be
	`easily implemented <https://wiki.haskell.org/Foldable_and_Traversable>`_ with a
	fold using ``Seq``'s `Foldable
	<https://wiki.haskell.org/Typeclassopedia#Foldable>`_ instance.

I am preparing a PR.

[sjshuck]

Just saw #1192

[meooow25]

Thanks for pointing out that bit of documentation, we should update it.

[sjshuck]

#1204