Split PurpleRequestField into its own files

Note that I `hg cp` the original files to preserve their history, so this _looks_ like a bunch of deleting, but it really isn't.

I've also added a bit of private API for the cross-struct access, but that may go away later. I did _not_ do any other API changes to make this a simpler review.

Testing Done:
Compiled, and opened Request Fields from the demo.

Reviewed at https://reviews.imfreedom.org/r/2311/