add negative bounds checks to accessors
more aggresive tests of getter bounds checking
more aggresive tests of setter bounds checking
disallow negative `sub`/`shift`

breaks compatibility: `sub` and `shift` cannot expand a sub-view any longer
test negative `sub`/`shift` is illegal
assure `of_bigarray` can't create invalid cstructs

breaks compatibility:
  * len and off can't point out outside of bigarray
  * if given, len is not clipped to size of the given bigarray
test `of_bigarray`
Merge pull request #33 from pqwy/bounds-everywhere

Bounds everywhere