WIP: Hard-deprecate, add ellipsis

[krlmlr]

This PR collects a few long-planned breaking changes:

• Hard-deprecates colonnade(), squeeze(), extra_cols() (Remove colonnade() #272, Remove extra_cols() and squeeze() #374)
  • see dm_disambiguate_cols() should optionally support the use RHS of column names for joins cynkra/dm#685 for the new API to customize pillar output
• Hard-deprecates expect_known_display() (Hard-deprecate expect_known_display() #460)
  • please use expect_snapshot()
• Adds an ellipsis to generics, for extensibility (closes Add ellipsis to tbl_sum(), type_sum(), obj_sum(), and size_sum() #250)
  • tbl_sum(), type_sum(), obj_sum(), size_sum()