Prefer packed switches over sparse switches when beneficial
Prefer packed switches over sparse switches when beneficial

Summary:
Pure goodness:
- Small uncompressed size
- Compresses better
- Better performance

Reviewed By: thezhangwei

Differential Revision: D14806629

fbshipit-source-id: 194983d9d7d9e40f76d9756831af5b931db06601

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex