Fix badness around fixed-size array decoding #29

The generated code for fixed size arrays is currently:

    let mut buf: [#ty; #value as usize] = unsafe { ::std::mem::uninitialized() };
    let sz = xdr_codec::unpack_array(input, &mut buf[..], #value as usize, None)?;
    (buf, sz)

This is unsafe because the array is uninitialized, and unpack_array will replace each field as it unpacks each element - ie, it will try to destruct the old (uninitialized) value.

The safe way to do this is with std::ptr::write, which simply overwrites the pointer without dropping it:

    fn uninit_ptr_setter<T>(p: &mut T, v: T) {
        unsafe { ::std::ptr::write(p as *mut T, v) }
    }
    let mut buf: [#ty; #value as usize] = unsafe { ::std::mem::uninitialized() };
    let sz = xdr_codec::unpack_array_with(input, &mut buf[..], #value as usize, uninit_ptr_setter, None)?;
    (buf, sz)

but this still has problems if unpack_array_with() fails and leaves the array partially initialized - it's Drop will end up trying to destruct uninitialized elements.