Support packed remote struct without destructuring

[dtolnay]

The destructuring-based implementation from #2079 is potentially problematic because not all objects can be destructured by value. For example:

#[repr(packed)]
pub struct RemotePackedNonCopy {
    pub a: u16,
    pub b: String,
}

#[derive(Deserialize)]
#[repr(packed)]
#[serde(remote = "RemotePackedNonCopy")]
pub struct RemotePackedNonCopyDef {
    a: u16,
    b: String,
}

impl Drop for RemotePackedNonCopyDef {
    fn drop(&mut self) {}
}

error[E0509]: cannot move out of type `RemotePackedNonCopyDef`, which implements the `Drop` trait
   --> test_suite/tests/test_gen.rs:876:10
    |
876 | #[derive(Deserialize)]
    |          ^^^^^^^^^^^
    |          |
    |          cannot move out of here
    |          data moved here
    |          move occurs because `__v1` has type `std::string::String`, which does not implement the `Copy` trait
    |
    = note: this error originates in the derive macro `Deserialize` (in Nightly builds, run with -Z macro-backtrace for more info)