Returns the packed size of the struct.
What is th packed size? Normally, struct members are aligned in a way that optimizes access speed. For example, on a 64bit platform, all members are aligned to start on a 64bit boundary https://doc.rust-lang.org/reference/type-layout.html.
Packed means what would be the size if no alignment would be used. In fact, packed size equals the sum of sizes of all members.