+cstruct.1.4.0

Comprehensive addition of bounds checking to all cstruct operations (from @pqwy in #33).  The major API-facing changes are:
* Disallow negative indexing with all cstruct accessors.
* Disallow negative `sub` and `shift` operations.
* Make sure `of_bigarray` cannot create invalid `cstruct` values.

This may break some older `cstruct` consumers that assumed that negative shifts were allowed, and so the version has been bumped to 1.4.0.