- 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

fix bug #707: optimised dict iteration over non-trivial expressions failed to evaluate the expression

--HG--
extra : rebase_source : b2f74c2b859a2e783da3c00cb0392e41f3ea0362

--HG--
extra : rebase_source : a4c51d10f4b7c88a994a0687202ac032a7baff33