Fix memory leaks in Context

[fedimser]

• Use weakref when passing make_callable and make_class to Interpreter, which breaks circular reference (Context -> Interpreter -> callback -> Context) and allows the Context object (and Interpreter) to be garbage collected.
• Do not register callables and classes for non-global Contexts in sys.modules. Use types.SimpleNamespace instead. Users access symbols for global Context via qdk.code, and symbols for other Contexts via context_variable.code.
  • This way we don't need to clear sys.modules when Context is garbage collected.
  • We still clear qdk.code when global context is invalidated on init().