extend transmute_ptr_to_ref to detect transmuting newtypes around ptrs

[oli-obk]

We just had something like the following code in IRC. It would be nice if clippy suggested to replace the transmute with &*(self.0 as *const i32)

struct Foo(*const i32);
fn foo(foo: &Foo) -> &i32 {
    unsafe { transmute(foo) }
}

This is an extension of the existing lint that complains about transmute<*const T, &T>(raw_ptr). We just need to check whether the transmuted-from type is a struct which has only a single raw pointer field (or all other fields are ZST).

[Jaic1]

This is an extension of the existing lint that complains about transmute<*const T, &T>(raw_ptr).

Is this lint (transmute_ptr_to_ref) the existing lint you mentioned? Also it seems like currently clippy will not lint for the example.