test interactions with butcher

[simonpcouch]

Closes #22.

Surprisingly, I couldn't find any examples of butchering before bundling resulting in errors. This PR adds tests for each of the bundle methods that directly wrap native serialization methods, as well as for the workflow setups described in #22, ensuring that we can butcher before bundling.

I didn't add a vignette, as the story seems to be simpler than I had anticipated, but open to ideas for some docs here if you think this ought to be called out explicitly somewhere. :)

This PR also includes changes to bundle.workflow that the lm + step_umap tests needed—currently, the workflows bundler only bundled the recipe specification, but also needs to bundled the trained recipe.

[juliasilge]

This looks great! I do think we need some docs somewhere at least saying you can do this. Two ideas I have:

• a roxygen template to add to the documentation pages for each bundle method that could be butchered, saying basically "No problem, use both, do bundle(butcher(x))"
• A short addendum to the existing vignette, saying that you could do bundle(butcher(keras_fit)) if appropriate. We don't actually support keras in butcher so maybe this one is worse.

[simonpcouch]

Love the idea on the butcher and bundle roxygen template. I didn't include it in h2o, keras, and torch docs, as it seems like we don't have/need butcher support for those!