Right now there is nothing wider than 24mm in native FE mount, though a 16-35mm f/4 will be here this year.
I was familiar with the 12mm Touit from work I did with it on the Fuji X1 last year, and think very highly of it.
As it turns out, the 12mm Tout covers mor than the APS-C format. In fact its coverage is closer to 1.3x than 1.5x. This means that on on A7r one gets about 16mm coverage and a 21M file. Just needs cropping out the corners.
Fantastic lens, sharp to the edge of coverage. Priced well, and there's nothing else for the A7 series that's as wide at the moment.