Refactor `output_templates`

[erlend-aasland]

Fairly early in output_templates(), we've got this comment:

        # we have to set these things before we're done:
        #    
        # docstring_prototype
        # docstring_definition
        # impl_prototype
        # methoddef_define
        # parser_prototype
        # parser_definition
        # impl_definition
        # cpp_if
        # cpp_endif
        # methoddef_ifndef

Post this comment, there are a few large blocks of code where most of these "fields" are set; these blocks depend on the type of call:

1. no args -- METH_NOARGS
2. one positional arg -- METH_O
3. optional groups
4. positional only args
5. methods/funcs with keyword args

Post these cases, most of the "fields" are set and there follows some fairly complex "post-processing" code (produce deprecation warnings, process some of the templates, and there's also some special handling for new/init methods.

We should try to refactor the five major cases in some kind of way. Perhaps a "builder class" that holds the required fields is the best approach. Perhaps "builder class" helpers for rendering Limited API fields and Internal API fields would be useful.