Make WTO iterating safer
Make WTO iterating safer

Summary:
Correct iteration of WtoComponents depends on the objects being in one
contiguous piece of memory. Make sure users don't accidentally copy these
objects. (moves are okay though)

Reviewed By: arnaudvenet

Differential Revision: D5878749

fbshipit-source-id: 4dcaa9b3c7cdad177f8ea7b659ad57839667a5e2

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