- shares and moves the functions [reduce_offset_by_validity] of Locations and
  Precise_locs into base.ml.
- replaces the boolean argument ~for_writing into the new type access, that
  represents Read, Write or No_access. Without any access, offsets must point
  into or just beyond the base validity.
- fixes the support of accesses of size 0: they are now invalid:
  + in bases with Invalid validity;
  + one past a base validity unless the base ends with an empty struct.