Removed `foldl` dependency

I decided to remove the `foldl` dependency for a few reasons:

* Writing custom folds required importing `Control.Foldl`

* It was an extra dependency

* `foldl` is probably going to be unstable in the near future

I made sure all the folds in `Pipes.Prelude` can still accept the fields of
a `Fold` or `FoldM` type.

It is more likely that I will make `foldl` depend on `pipes` to provide
`Fold` integration for `pipes`.