Foldable Conversions

This is related to some of what I brought up in https://github.com/haskell/containers/issues/338. I was just working with Data.Set and wanted to call unions on some foldable collection of Sets. I can just write:

```
unions . toList
```

And that works fine. And for some data structures, the intermediate list may even get fused away. Maybe. But any time I have to call toList on a Foldable, I'm unsatisfied because whatever cosumes the list could have been written to just fold over the collection instead. It seems like generalizing unions should be possible (although it does use a special variant of foldl).

Additionally, having a fromFoldable would be nice. It would be misleading to generalize fromList since the name would then be misleading.

But for the most part, I would like the see the functions whose names don't explicitly mention the word "list" be generalized, with rewrite rules to use the tunes list variants when possible.