CLN/DEPR: Consistent naming for isnull, notnull, fillna, dropna
In 2.0, these functions could be named more consistently, either all `*na` or `*null`. Or, to preserve backwards compatibility, `isna`, `notna`, `fillnull` and `dropnull` could be added as aliases. Either way, it would be helpful to take the guess work out of this.

Let me know if this is not the correct forum for such proposals.