std::function makes code harder to debug because it requires stepping
through a separate destructor and call operator.  It's use unnecessary
in the Parser since the functions taking functors are private and are
only used within idl_parser.cpp.  Therefore the definitions can stay in
idl_parser.cpp as well.  Only care must be taken that the definitions
appear before use but that's already true and all compilers will
complain equally if it get's violated.  This change might also improve
performance since it might allow inlining where it wasn't possible
before but I haven't measured that.