dav1d 0.5.1 'Asiatic Cheetah', the fast and small AV1 decoder

This is a minor update of the 0.5.0 version of dav1d, the fast and small AV1 decoder,
codename 'Asiatic Cheetah'.

It supports all the AV1 features and all bitdepths.

0.5.1 brings improvements in speed for SSE2 CPUs (up to 50% speedup), and
ARMv7 CPUs (up to 41% speedup).
It also fixes minor issues and minor speed improvements for other