custom op checking attached to subroutines

 Devel::CallChecker makes some new features of the Perl 5.14.0 C API
 available to XS modules running on older versions of Perl.
 The features are centred around the function cv_set_call_checker,
 which allows XS code to attach a magical annotation
 to a Perl subroutine,
 resulting in resolvable calls to that subroutine
 being mutated at compile time by arbitrary C code.
 This module makes cv_set_call_checker
 and several supporting functions available.
 (It is possible to achieve the effect of cv_set_call_checker
 from XS code on much earlier Perl versions,
 but it is painful to achieve without the centralised facility.)