Support rkyv 0.8.x

[ruseinov]

Unfortunately the recent nalgebra release does not support rkyv = 0.8 as it still depends on 0.7.

There are a number of breaking changes in terms of derive macros, attributes and the api.

[zyansheep]

Waiting on https://github.com/rust-num/num-complex/pull/142#issuecomment-3169073590 :/