Summary:
Refactors MapBuffer-related `primitives.h` to be namespaced with `MapBuffer` class, to avoid name collisions and confusion later on.
Most of the size constants are moved to relevant .cpp files or updated to use `sizeof`.
Additionally, adds a little bit of documentation about `MapBuffer` serialization format.
Changelog: [Internal]
Reviewed By: mdvacca
Differential Revision: D33662487
fbshipit-source-id: 5a7a2b1c7f2bb13ee1edfc5fae51ba88c34f0d3c
3c86e82a02