Revert readonly_state; fix is fully in clear_deleteme_list

Greenlet construction (green_new, green_unswitchable_new) must also
trigger clear_deleteme_list because existing code and tests depend
on it for cross-thread cleanup (e.g. test_dealloc_other_thread uses
RawGreenlet() to trigger deleteme processing).

With std::swap + exception preservation in clear_deleteme_list,
the crash and exception-loss bugs are fixed at the source.
The readonly_state / const borrow_current utilities are no longer
needed and are removed.

Made-with: Cursor

Author: nbouvrette

src/greenlet/PyGreenlet.cpp

@@ -60,7 +60,7 @@ green_new(PyTypeObject* type, PyObject* UNUSED(args), PyObject* UNUSED(kwds))
         // C++ object assigns it to the pimpl pointer of the Python
         // object (o); we'll need that later.
         UserGreenlet* c = new UserGreenlet(o,
-                              GET_THREAD_STATE().readonly_state().borrow_current());
+                              GET_THREAD_STATE().state().borrow_current());
         assert(Py_REFCNT(o) == 1);
         assert(c == o->pimpl);
     }

src/greenlet/PyGreenletUnswitchable.cpp

@@ -50,7 +50,7 @@ green_unswitchable_new(PyTypeObject* type, PyObject* UNUSED(args), PyObject* UNU
     PyGreenlet* o =
         (PyGreenlet*)PyBaseObject_Type.tp_new(type, mod_globs->empty_tuple, mod_globs->empty_dict);
     if (o) {
-        new BrokenGreenlet(o, GET_THREAD_STATE().readonly_state().borrow_current());
+        new BrokenGreenlet(o, GET_THREAD_STATE().state().borrow_current());
         assert(Py_REFCNT(o) == 1);
     }
     return o;

src/greenlet/TThreadState.hpp

@@ -257,15 +257,6 @@ class ThreadState {
         return this->current_greenlet;
     }
 
-    /**
-     * As for const get_current(), but returns a borrowed reference.
-     * Does no maintenance.
-     */
-    inline BorrowedGreenlet borrow_current() const
-    {
-        return this->current_greenlet;
-    }
-
     template<typename T, refs::TypeChecker TC>
     inline bool is_current(const refs::PyObjectPointer<T, TC>& obj) const
     {

src/greenlet/TThreadStateCreator.hpp

@@ -75,21 +75,6 @@ class ThreadStateCreator
         return *this->_state;
     }
 
-    /**
-     * Return the ThreadState as a const reference, which restricts the
-     * caller to read-only operations (the const overloads of
-     * get_current() and borrow_current()).
-     *
-     * Those const overloads skip the clear_deleteme_list() maintenance
-     * that the non-const versions perform.  Use this when the caller
-     * only needs the current greenlet reference and triggering
-     * cross-thread cleanup is unnecessary (e.g. greenlet construction).
-     */
-    inline const ThreadState& readonly_state()
-    {
-        return this->state();
-    }
-
     operator ThreadState&()
     {
         return this->state();