fix decorator lookup by avoiding (re-)assignments to the function/class name before their execution (ticket #593)

CyFunction

- generate correct code for optimised override calls to methods of builtin types in subtypes
- optimise calls for 'final' subtypes
- disable optimised C-API calls for non-final subtypes

fix support for redeclaring builtin types as external extension types, add a big FIXME to mark for eventual removal

Silence warning about unused args