inspection methods for test output

[corybrunson]

Test output, e.g. from permutation_test(), currently returns a somewhat unwieldy list. I think the following methods would be useful to implement:

• print() (see various methods for {stats} test output for inspiration)
• summary(), possibly
• tidy() and glance() from {generics}, as used in {broom} and its extensions
• autoplot(), e.g. histograms of null samples to illustrate p-values

However, this requires that the tests return objects of some S3 class. This could be a new class or classes, or the existing 'htest' class---or possibly both, in case some tasks can be dispatched to 'htest' methods but others must be more specific.