API: add top-level functions as method
xref https://github.com/pydata/pandas/pull/12578

so its pretty easy to add (and eventually deprecate) certain top-level functions that could/should be methods. Promotes a cleaner syntax and method chaining.

In 0.18.1 would propose adding these, deprecating in 0.19.0.

This would eliminate the ability to use directly with `np.ndarrays`, but I view this is as a positive (note we effectively did this for the `.rolling/expanding/ewm` with `np.ndarrays` as well, though deprecated for a while).
- [ ] `pd.crosstab` (DataFrame)
- [x] `pd.melt` (DataFrame) (PR #15521); added to ``DataFrame``
- [ ] `pd.get_dummies` (both)
- [ ] `pd.cut/qcut` (only on Series)
