Function get_trailer_offset 

const fn get_trailer_offset(
    header_size: usize,
    core_size: usize,
    core_align: usize,
    trailer_align: usize,
) -> usize

Compute the offset of the Trailer field in Cell<T, S> using the #[repr(C)] algorithm.

Pseudo-code for the #[repr(C)] algorithm can be found here: https://doc.rust-lang.org/reference/type-layout.html#reprc-structs