Simplify object creation with OBJ_NEW_BEGIN() macro.

Eliminate the boilerplate of declaring this and assigning memory to it, which is the same for the vast majority of object creations.

Keep the old version of the macro as OBJ_NEW_BASE_BEGIN() for a few exceptions in the core code and (mostly) in the tests.

src/postgres/client.c

@@ -53,12 +53,8 @@ pgClientNew(const String *host, const unsigned int port, const String *database,
     ASSERT(port >= 1 && port <= 65535);
     ASSERT(database != NULL);
 
-    PgClient *this = NULL;
-
     OBJ_NEW_BEGIN(PgClient, .childQty = MEM_CONTEXT_QTY_MAX, .callbackQty = 1)
     {
-        this = OBJ_NEW_ALLOC();
-
         *this = (PgClient)
         {
             .pub =