Refactor the SpeedSettings impl
Refactor the SpeedSettings impl

The impetus for this change is that the documentation for speed
settings, being a manually-maintained comment, would easily become out
of date and was difficult to maintain. This change makes it such that
rather than having a separate function for each speed setting to
determine when to enable it, the `from_preset` function now sets all
settings per speed level, with the `default()` being speed 0, and
overriding as the speed increases. The idea is that this will make it
much easier to see which features are enabled at which speed levels.

xiph/rav1e,xiph/rav1e