Support unboxed opaque types that are `Copy`

It would be cool if there was support for safer_ffi opaque types that don't require boxing because they are Copy. Ostensibly, one could just look at the size and alignment of the type and create a C type with the same layout. This is what autocxx does and it seems to work well:

Generated rust code:

#[repr(C, align(4))]
pub struct HmdMatrix34_t {
    _pinned: core::marker::PhantomData<core::marker::PhantomPinned>,
    _non_send_sync: core::marker::PhantomData<[*const u8; 0]>,
    _data: [u8; 48],
}

Corresponding C/C++ POD type:

struct HmdMatrix34_t
{
    float m[3][4];
};

I'm assuming that this solution won't work for non-copy types (implying that they have a drop fn and cannot be memcopied), but in my API I have a bunch of opaque types that are Copy, and I'l really like to avoid boxing all of them, and having the caller have to allocate and drop them all the time

getditto / safer_ffi

Support unboxed opaque types that are `Copy` #103