* Move ifndef_symbols, includes and add_include() from Clinic to
Codegen. Add a 'codegen' (Codegen) attribute to Clinic.
* Remove libclinic.crenderdata module: move code to libclinic.codegen.
* BlockPrinter.print_block(): remove unused 'limited_capi' argument.
Remove also 'core_includes' parameter.
* Add get_includes() methods.
* Make Codegen.ifndef_symbols private.
* Make Codegen.includes private.
* Make CConverter.includes private.
Add libclinic.clanguage module and move the following classes and
functions there:
* CLanguage
* declare_parser()
Add libclinic.codegen and move the following classes there:
* BlockPrinter
* BufferSeries
* Destination
Move the following functions to libclinic.function:
* permute_left_option_groups()
* permute_optional_groups()
* permute_right_option_groups()