Use fixed-sized array in luma/chroma pred mode counts

[redzic]

According to #2236, this was converted to an ArrayVec since at that time Rust did not support deriving Debug for fixed-sized arrays above 32 elements, but this is no longer the case and thus ArrayVec is not needed.

[redzic]

Hmm, it builds locally for me, except when using --features=serialize. I'll look into it further

[barrbrain]

We might need to supplement with https://github.com/est31/serde-big-array to cover --features=serialize.