Include rav1e in release binaries

[gitoss]

The rav1e lib seems to be quite competetive vs. libaom-av1 for still image encoding - it would be nice to have it included in release binaries.

There are up-to-date binaries on AppVeyor, but they don't include rav1e yet either...

[joedrago]

I've enabled rav1e on AppVeyor builds, so rav1e encoding should be available in the next release. This turns a 9 minute build into a 28 minute build, but it is worth it for now. Someday I'll learn how to use cargo more efficiently and it won't take so long!

[1480c1]

It might be worth looking into setting up sccache (I personally haven't gotten it to work in good capacity on windows, so I'm not sure if I can help with it)