The inheritance of raw reborrowing is not (no longer) necessary. Let's remove it and call it shorten.

Get rid of down_close in creation.v : we can take all the lifetimes that are alive, which is simpler.

Instead, we strengthen a little bit the induction hypothesis.

Put the shift_loc notation in the right scope so that it is parsed/printed properly even when appearing in expressions.

The common principle is "unnesting an index borrow". This simplifies
the proofs of reborrowing and unnesting, and, moreover, gives a slightly
stronger unnesting rule (the masks are different).