inconsistent usage of function pointer typedefs

[sebastinas]

The us of the function pointer typedef 'convertible_function' is inconsistent in boost/python/converter/registry.hpp and libs/python/converter/registry.cpp.

In registry.hpp 'insert' is defined as 'void insert(void* (*convert)(PyObject*), type_info)' whereas it's 'void insert(convertible_function, type_info)' in registry.cpp. The same applies to 'void insert(convertible_function, constructor_function, type_info)' and 'void push_back(convertible_function, constructor_function, type_info)'.

The attached patch replaces the function pointers with the appropriate typedefs.