Build the slice directly in array_chunks_mut
Build the slice directly in array_chunks_mut

Review discussion found that the concern about aliasing was overblown,
so we can simplify this to cast from one slice to another directly.

graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust