Change the notation for borrows : now, it is just a notation for the predicate transformer (it does not include the payload of the borrow).
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--- The new behavior is somewhat more intuitive, and better behaved wrt view predicates.

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).