API: DataFrame.melt(), DataFrame.wide_to_long()
xref https://github.com/pandas-dev/pandas/pull/14779

So I think it might be nice to have these as convenience methods on DataFrame().
we can deprecate (``pd.melt`` & ``pd.wide_to_long``), though we *didn't* deprecate ``pd.pivot_table``, even though it exists on ``DataFrame``, so this can be a discussion point.

but if we do this, then maybe have a shorter / better name than ``wide_to_long``? (even though it is strictly speaking pretty informative).

