gh-111178: Fix function signatures in longobject.c (#124895)

* Add _PyLong_CAST() macro.
* Move forward declarations to the top of longobject.c.
* Change long_add(), long_sub(), long_mul(), long_neg(),
  long_lshift(), long_abs() to take PyLongObject* and return
  PyLongObject*. Avoid CHECK_BINOP() test.
* Add long_add_method(), long_sub_method(), long_mul_method(),
  long_neg_method(), long_lshift_method(), and long_abs_method()
  which take PyObject* and return PyObject*. Implement CHECK_BINOP()
  test.
* Add long_lshift_int64() function.
* _PyLong_DivmodNear() calls long_lshift_int64(obj, 1) instead of
  long_lshift_obj(obj, one).

Include/cpython/longobject.h

@@ -2,6 +2,9 @@
 #  error "this header file must not be included directly"
 #endif
 
+#define _PyLong_CAST(op) \
+    (assert(PyLong_Check(op)), _Py_CAST(PyLongObject*, (op)))
+
 PyAPI_FUNC(PyObject*) PyLong_FromUnicodeObject(PyObject *u, int base);
 
 #define Py_ASNATIVEBYTES_DEFAULTS -1