Fix the .shorten() method in SerdeDataStructWrap

[sffc]

There is the following method on SerdeSeDataStructWrap:

impl<'d, 's: 'd> SerdeSeDataStructWrap<'d, 's> {
    fn shorten(self) -> SerdeSeDataStructWrap<'d, 'd> {
        // This is safe because 's exceeds 'd
        // TODO(#760): The types must be covariant for this to actually be safe.
        unsafe { std::mem::transmute(self) }
    }
}

This is a follow-up from #745 (comment) to re-work this code so it doesn't need unsafe.

Blocked on #752

[sffc]

Should be obsolete based on #752.

Update: It doesn't appear that #752 made this obsolete.

[sffc]

Not completely fixed; still need #1041