Statically find msgSend functions on apple devices

[madsmtm]

The functions are found by looking at the Encode::ENCODING and size of the return type.

This ensures that we don't incur a performance overhead for that (though we probably didn't before anyways).

This is also more correct since it allows the following:

#[repr(transparent)]
struct Wrapper {
    inner: f32,
}
unsafe impl Encode for Wrapper {
    const ENCODING: Encoding<'static> = f32::ENCODING;
}

To use the correct msgSend implementation.