Summary: `F14Vector{Map,Set}` have specified iteration order: inverse insertion order. When serialized, however, the order does not round-trip (in fact, it flips). This diff introduces an extension point to specify the iteration order for serialization, and it specializes it for F14 to make the iteration order round-trip.

Reviewed By: vitaut

Differential Revision: D17108774

fbshipit-source-id: 466b746baacff861242729bfa62447c138094cee