Function get_core_offset 

const fn get_core_offset(header_size: usize, core_align: usize) -> usize

Compute the offset of the Core<T, S> 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