rkyv-08 feature

[Awpteamoose]

Supersedes #1614

Not sure what to do about CI as currently the features are incompatible with rkyv 0.7, but I could use fully qualified paths for rkyv_08::Archive etc and then it should be ok?

Alternatively, could do smth like

// lib.rs
#[cfg(any(feature = "rkyv", feature = "rkyv-16", feature = "rkyv-32", feature = "rkyv-64"))]
pub(crate) use rkyv_07 as rkyv;

#[cfg(any(feature = "rkyv-08", feature = "rkyv-08-16", feature = "rkyv-08-32", feature = "rkyv-08-64"))]
pub(crate) use rkyv_08 as rkyv;

// elsewhere
use crate::rkyv::{Archive, Serialize, Deserialize};