Summary:
The pushmi library has grown to the point where a little more structure would aid comprehensibility.

Also, finer-grained headers and build targets would make it easier for downstream projects to include only what they absolutely need.

Reviewed By: kirkshoop

Differential Revision: D14802895

fbshipit-source-id: 8d253f97c5d0529530cc3b7110505000f513ce83