Prevent move-out on pointer holding structs

[R1kM]

As described in the title. There currently is some special-casing to avoid move-out when, e.g., accessing an array of non-Copy elements.
This PR extends what is considered as a non-Copy element to also consider pointer holding structs.

[msprotz]

I take it pointer_holding_structs only contains mutable pointers? I guess we don't know at that stage so we have to be conservative...

[R1kM]

Actually, since we do not derive the Copy or Clone traits for structures, the check should probably be more restrictive, and check for any struct type.

For instance, despite using a struct that does not contain pointers, the following Rust code does not compile

struct T {
    x: u32,
    y: u32
}

fn g(s: &[T]) {
    let x: T = s[0];
}

Coming back to a simplified version of the HACL-rs example that inspired this PR, the following code would indeed work without the additional reference added by karamel, while taking a mutable pointer in struct T would lead to a compilation failure.

#[derive(Copy, Clone)]
struct T <'a> {
    x: u32,
    y: &'a [u32]
}

fn g(s: &[T]) {
    let x: T = s[0];
}

However, adding support for Clone/Copy on generated struct types does not seem needed at the moment, and this conservative pattern is sufficient for compilation. I would suggest to revisit only if needed.