rapid evolution towards producing real .pyc files (even though I don't

rapid evolution towards producing real .pyc files (even though I don't
handle most of the language syntax yet)

create NestedCodeGenerator used to generator the separate code object
that needs to be passed as an argument to MAKE_FUNCTION when a def
stmt is found (probably useful for class too)

change CodeGenerator.visitFunction to use the NestedCG

add CompiledModule class to handle creation of .pyc (pretty minimal
for now)

add makeCodeObject method to PythonVMCode that replaces symbolic names
with indexes into slots of the code code.  the design of this
class will probably need to be revised.