As before, this uses hg cp, so it looks like a bunch of deletion.
Again, there's a bit of private API for the cross-struct access, which should go away later.
I also dropped 'field' from the file name, as it seems like there's no need to have that there in the final class name.
I did not do any other API changes to make this a simpler review.

Compiled and opened Request Fields from the Demo.