Materialize code constants through the VM (#8867)

PyObjBag built code object constants from a Context alone, and its
frozenset arm was unimplemented!(), so ctx.new_code() panicked on code
containing a frozenset constant such as `x in {1, 2, 3}`.

IntoCodeObject now takes a VirtualMachine and uses PyVmBag for bytecode
and frozen code objects. Context::new_code, PyObjBag and
AsBag for &Context are removed; callers use VirtualMachine::new_code.

Assisted-by: Claude:claude-opus-5-5
Assisted-by: Grok:grok-4.7

examples/freeze/main.rs

@@ -11,7 +11,7 @@ fn run(vm: &vm::VirtualMachine) -> vm::PyResult<()> {
     // the file parameter is relative to the current file.
     let module = vm::py_compile!(file = "freeze.py");
 
-    let res = vm.run_code_obj(vm.ctx.new_code(module), scope);
+    let res = vm.run_code_obj(vm.new_code(module), scope);
 
     if let Err(exc) = res {
         vm.print_exception(&exc);

examples/mini_repl.rs

@@ -16,7 +16,7 @@ macro_rules! add_python_function {
         // compile the code to bytecode
         let code = vm::py_compile!(source = $src);
         // convert the rustpython_compiler_core::CodeObject to a PyRef<PyCode>
-        let code = $vm.ctx.new_code(code);
+        let code = $vm.new_code(code);
 
         // run the python code in the scope to store the function
         $vm.run_code_obj(code, $scope.clone())