[ENH] dropna parameter for pivot_longer (#1136)

* dump down to dictionary for multiple values_to, for performance

* minor fixes

* move column reordering to _final_frame_longer

* dropna for simple melting - no .value

* dropna for names_pattern_sequence

* implement dropna in a single location

* add tests for dropna

* more tests; faster build for _base_melt

* changelog

* cleanup

* proper renaming of variables

Author: samukweku

CHANGELOG.md

@@ -13,6 +13,7 @@
 -   [ENH] Added support for extension arrays in `expand_grid`. Issue #1121 @samukweku
 -   [ENH] Add `names_expand` and `index_expand` parameters to `pivot_wider` for exposing missing categoricals. Issue #1108 @samukweku
 -   [ENH] Add fix  for slicing error when selecting columns in `pivot_wider`. Issue #1134 @samukweku
+-   [ENH] `dropna` parameter added to `pivot_longer`. Issue #1132 @samukweku
 
 ## [v0.23.1] - 2022-05-03