gh-106812: Small stack effect fixes (#107759)

- Generalize the syntax for the type of a stack effect to allow a trailing `*`,
  so we can declare something as e.g. `PyCodeObject *`.

- When generating assignments for stack effects,
  the type of the value on the stack should be the default (i.e., `PyObject *`)
  even when the variable copied to/from it has a different type,
  so that an appropriate cast is generated
  However, not when the variable is an array --
  then the type is taken from the variable (as it is always `PyObject **`).