Generate exceptions files so they include all Python exceptions

Had to flatten CMake structures to get the dependencies working
for the generated files; should maybe fix this someday, but a flat
structure is just fine for now.
Split sequences into separate file and add list to builtins.
Add slice, more functionality for list, tuple
Made builtins inherit from object
type now inherits from object
Make builtin ctors from object implicit
Add wrappers for getattr, setattr, delattr
Make numeric conversions explicit