Update folding proxies to use WriterP

* The various folding proxies from `Control.Proxy.Prelude.Base` have been updated to use `WriterP` instead of `WriterT`. 

* `foldlD'` and `foldU'`, which relied on `StateT`, were deleted, since they are redundant now that `foldD` and `foldU` are strict, a property that is mentioned in their documentation now. 

* There is no need for `StateT` or `WriterT` in `Control.Proxy.Prelude.Base`, so their modules have been removed from the imports and exports.

* `Control.Proxy.Prelude.Base` now exports: `WriterP`, `runWriterP`, `runWriterK`, `execWriterP` and `execWriterK`.

* The tutorial section about “Folds” has been updated to only mention folding using `WriterP`.