transition from `add_tailor(prop)` and `method` to `fit.workflow(calibration)`

[simonpcouch]

Closes #255, closes #254, closes #252, closes #233, and related to this gist.

This PR removes the add_tailor(prop) and method arguments in favor of an argument fit.workflow(calibration) giving the data to fit the calibrator on.

Benefits:

• The workflow now truly has enough information to fit without leaking data (method wasn't actually enough)
• add_tailor(method) really wasn't truly independent of the data/resampling scheme
• workflows does not need to "know about" rsample

Thanks @hfrick for this suggestion—this feels much better.

This PR should be much easier to review commit-by-commit than altogether.

TODOs from here: update tune and (possibly?) rename .should_inner_split() to something like .workflow_needs_calibration().