Remove the option to have nested scopes or old LGB scopes.  This has a
large impact on the code base, by removing the need for two variants
of each CodeGenerator.

Add a get_module() method to CodeGenerator objects, used to get the
future features for the current module.

Set CO_GENERATOR, CO_GENERATOR_ALLOWED, and CO_FUTURE_DIVISION flags
as appropriate.

Attempt to fix the value of nlocals in newCodeObject(), assuming that
nlocals is 0 if CO_NEWLOCALS is not defined.