bpo-39542: Simplify _Py_NewReference() (GH-18332)

* Remove _Py_INC_REFTOTAL and _Py_DEC_REFTOTAL macros: modify
  directly _Py_RefTotal.
* _Py_ForgetReference() is no longer defined if the Py_TRACE_REFS
  macro is not defined.
* Remove _Py_NewReference() implementation from object.c:
  unify the two implementations in object.h inline function.
* Fix Py_TRACE_REFS build: _Py_INC_TPALLOCS() macro has been removed.
diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index 87f88ab..ae6b50f 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -311,7 +311,9 @@
 static inline void
 dictkeys_incref(PyDictKeysObject *dk)
 {
-    _Py_INC_REFTOTAL;
+#ifdef Py_REF_DEBUG
+    _Py_RefTotal++;
+#endif
     dk->dk_refcnt++;
 }
 
@@ -319,7 +321,9 @@
 dictkeys_decref(PyDictKeysObject *dk)
 {
     assert(dk->dk_refcnt > 0);
-    _Py_DEC_REFTOTAL;
+#ifdef Py_REF_DEBUG
+    _Py_RefTotal--;
+#endif
     if (--dk->dk_refcnt == 0) {
         free_keys_object(dk);
     }
@@ -563,7 +567,9 @@
             return NULL;
         }
     }
-    _Py_INC_REFTOTAL;
+#ifdef Py_REF_DEBUG
+    _Py_RefTotal++;
+#endif
     dk->dk_refcnt = 1;
     dk->dk_size = size;
     dk->dk_usable = usable;
@@ -687,10 +693,12 @@
     }
 
     /* Since we copied the keys table we now have an extra reference
-       in the system.  Manually call _Py_INC_REFTOTAL to signal that
+       in the system.  Manually call increment _Py_RefTotal to signal that
        we have it now; calling dictkeys_incref would be an error as
        keys->dk_refcnt is already set to 1 (after memcpy). */
-    _Py_INC_REFTOTAL;
+#ifdef Py_REF_DEBUG
+    _Py_RefTotal++;
+#endif
 
     return (PyObject *)new;
 }
@@ -1249,13 +1257,15 @@
 
         assert(oldkeys->dk_lookup != lookdict_split);
         assert(oldkeys->dk_refcnt == 1);
+#ifdef Py_REF_DEBUG
+        _Py_RefTotal--;
+#endif
         if (oldkeys->dk_size == PyDict_MINSIZE &&
-            numfreekeys < PyDict_MAXFREELIST) {
-            _Py_DEC_REFTOTAL;
+            numfreekeys < PyDict_MAXFREELIST)
+        {
             keys_free_list[numfreekeys++] = oldkeys;
         }
         else {
-            _Py_DEC_REFTOTAL;
             PyObject_FREE(oldkeys);
         }
     }