enum Av1BlockIntraInter: Remove #[repr(C)], reducing size by 4 by…

…tes (#1333)

This is a simpler, fully safe alternative to #1331. It reduces
`Av1BlockIntraInter` from 28 to 24 bytes, and `Av1Block` from 36 to 32
bytes, just like #1331.

I haven't checked memory usage, though, since I'm not sure how to do
that, but since it reduces the type sizes the same, I assume it would
have the same memory savings.

src/levels.rs

@@ -532,7 +532,6 @@ pub struct Av1BlockInter {
     pub tx_split1: u16,
 }
 
-#[repr(C)]
 pub enum Av1BlockIntraInter {
     Intra(Av1BlockIntra),
     Inter(Av1BlockInter),