add struct->struct-lens

[AlexKnauth]

see #267

[jackfirth]

This is meant to be an isomorphism yes? I think allowing field reordering would be good. A contrived use case I can think of off the top of my head is wanting a lens to view a position struct in reverse order:

(struct position (x y))
(define inverse-pos-lens (struct->struct-lens position position [x y] [y x]))
(lens-view inverse-pos-lens (position 1 2))

[AlexKnauth]

What you said in (#267 (comment)) about lens-join/struct made me think that of the cases where you might want to re-order the fields, a lot of those could be cases where you might want the freedom and power of (lens-join/struct position position-y-lens position-x-lens). I can see those use cases, but I can see many more where you might want (lens-join/struct position (lens-compose negate-lens position-y-lens) position-x-lens) or something similar.

[jackfirth]

Why is the #%app needed?

[AlexKnauth]

I'm not sure, but I suspect it's because the constructor-id from the struct-info ends up pointing to an identifier macro or something like that that messes it up in some way. I forget.