Update structs to remove 'zero type structs' for consumers.

[rthomas]

This is related to servo/core-text-rs#62 to remove the warnings when building.

The offending types for core-text are CGAffineTransform and CGContextRef.

Other warnings in the crate should also be cleaned up or silenced. The method for the two listed types can be found at rust-lang/rust#27303

[rthomas]

@jdm I've raised this to track the work for this crate.