add add-lens, subtract-lens, and modulo-lens

[AlexKnauth]

I still need to provide it from unstable/lens and add it to the unstable docs.

> (lens-view (add-lens 5) 3)
8
> (lens-set (add-lens 5) 3 8)
3
> (lens-set (add-lens 5) 3 27)
22
> (lens-view (modulo-lens 10) 2)
2
> (lens-set (modulo-lens 10) 2 8)
8
> (lens-view (modulo-lens 10) 23)
3
> (lens-set (modulo-lens 10) 23 9)
29

Resolves #288

[coveralls]

Coverage increased (+0.01%) to 98.937% when pulling de41ec0 on AlexKnauth:data-number into 11ecc2f on jackfirth:master.

[jackfirth]

add-lens and subtract-lens don't seem as useful, since they're just isomorphisms that don't do any destructuring.

This has me thinking about #205 again. Since that issue was last discussed, I've run into cases where I want just isomorphisms and don't care about lenses. As we add more utility isomorphisms like add and subtract, there will be more use in having isomorphisms separate.

[AlexKnauth]

I just ran into a situation where a multiply-lens (for non-zero scalar inputs) would be useful. I wanted a round-to-the-nearest-thousandth function and I wanted

(define round-to-the-nearest-thousandth (lens-transform (multiply-lens 1000) _ exact-round))