Rewrite rav1d-tools in safe Rust from scratch

[kkysen]

These two tools, dav1d (which we'll maybe rename to rav1d) and seek-stress, are fairly small (compared to the rest of librav1d), but they do a lot of unsafe type casting (simulating virtual classes) that is difficult to straightforwardly make safe in Rust. Also, for some of the code in these tools, there are already existing Rust libraries we can reuse, like mp4parse (see closed #16). And finally, we can use the Rust API directly here, making things much safer and eliminating the DRav1d overhead of storing duplicate dav1d types.