Removal of the unsafe index operators

The standard index operators are now using new compiler primitives:
e.g  "%array_indexop_get", "%string_indexop_set", "caml_ba_indexop_set_1".
Depending on the "-unsafe" compiler options, these new primitives
are then translated to the corresponding safe and unsafe version.
Due to these changes, the unsafe index operators are no longer
needed.

The documentation of the Bigarray and Pervasives interface file have also
been slightly improved.