I kept it building the most straight-forward way possible (pub use) because it
seems to me that bindings is not a bad name, and we should probably move
structs.rs to be bindings.rs rather than the other way around.
But that's a different bug in any case, need to think more about it.
Differential Revision: https://phabricator.services.mozilla.com/D24713
Majority of build_gecko.rs is just the straightforward conversion from
regen.py. There are two differences that:
1. Side in whitelist is changed to mozilla::Side
2. std::atomic__My_base is added to opaque types for Windows