Unfortunately funty::Numeric cannot require/guarantee that Self::Bytes is actually [u8; N], because it's not (yet?) allowed to write a signature like -> [u8; Self::BYTES] (where BYTES is an associated const: usize item on the trait). The general bound you probably want to place on Self::Bytes is probably AsRef<[u8]>; the trait is implemented for all [T; N] and allows you to call .as_ref() to get &[T].
The trait you actually want for T, though, is probably bytemuck::Pod (for "plain ol' data"). This will allow you to use cast_slice to directly go from &[T] to &[u8] without any copying.