module for safely calling isa and friends on things that may not be objects

 Since Perl allows providing a subroutine reference, or a method name, to
 the -> operator (when used as a method call) and a subroutine doesn't require
 the invocant to actually be an object, Safe::Isa can be used to create safe
 versions of isa, can, does and DOES by using a subroutine reference that
 only tries to call the method if it's actually used on an object.