Ensure a side vector's `object` and `VECTORSIDE` are consistent

A side vector belongs to one or two elements. One of them is stored as a
representative in the vector's `object` member and the side of the
`object` it belongs to is stored in `VECTORSIDE` as part of the control
word.

When restoring consistency in `ElementObjMkCons` this reverse link from
the side vector to the element is restored, but the `VECTORSIDE` was
not. This can lead to an inconsistent view for side vectors belonging to
two elements:

On the master, let the vector `v`'s representative element be `A` and
the side of the element be `a`. Let `B` (`b`) be the other
representative and assume `a ≠ b`. As the vector's control word is
global data (`EL_GDATA`), the non-master side vectors will also have
`VECTORSIDE` set to `a`.

If now `ElementObjMkCons` is called first for `A` and then for `B`, then
`v.object` will first be set to `A`, but then `B`. However `VECTORSIDE`
is still `a`!

This change makes sure that `VECTORSIDE` is also updated alongside the
`object` pointer. Note that it is (still) not guaranteed that the same
representative is chosen.

Closes #12