huge update:

introduced meta tasks which are used by `_map()`;
introduced task groups;
ending with `gather_and_close()` now;
pool unittests rewritten accordingly;
two new helper classes

src/asyncio_taskpool/group_register.py

@@ -0,0 +1,75 @@
+__author__ = "Daniil Fajnberg"
+__copyright__ = "Copyright © 2022 Daniil Fajnberg"
+__license__ = """GNU LGPLv3.0
+
+This file is part of asyncio-taskpool.
+
+asyncio-taskpool is free software: you can redistribute it and/or modify it under the terms of
+version 3.0 of the GNU Lesser General Public License as published by the Free Software Foundation.
+
+asyncio-taskpool is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; 
+without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. 
+See the GNU Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public License along with asyncio-taskpool. 
+If not, see <https://www.gnu.org/licenses/>."""
+
+__doc__ = """
+This module contains the definition of the `TaskGroupRegister` class.
+"""
+
+
+from asyncio.locks import Lock
+from collections.abc import MutableSet
+from typing import Iterator, Set
+
+
+class TaskGroupRegister(MutableSet):
+    """
+    This class combines the interface of a regular `set` with that of the `asyncio.Lock`.
+
+    It serves simultaneously as a container of IDs of tasks that belong to the same group, and as a mechanism for
+    preventing race conditions within a task group. The lock should be acquired before cancelling the entire group of
+    tasks, as well as before starting a task within the group.
+    """
+
+    def __init__(self, *task_ids: int) -> None:
+        self._ids: Set[int] = set(task_ids)
+        self._lock = Lock()
+
+    def __contains__(self, task_id: int) -> bool:
+        """Abstract method for the `MutableSet` base class."""
+        return task_id in self._ids
+
+    def __iter__(self) -> Iterator[int]:
+        """Abstract method for the `MutableSet` base class."""
+        return iter(self._ids)
+
+    def __len__(self) -> int:
+        """Abstract method for the `MutableSet` base class."""
+        return len(self._ids)
+
+    def add(self, task_id: int) -> None:
+        """Abstract method for the `MutableSet` base class."""
+        self._ids.add(task_id)
+
+    def discard(self, task_id: int) -> None:
+        """Abstract method for the `MutableSet` base class."""
+        self._ids.discard(task_id)
+
+    async def acquire(self) -> bool:
+        """Wrapper around the lock's `acquire()` method."""
+        return await self._lock.acquire()
+
+    def release(self) -> None:
+        """Wrapper around the lock's `release()` method."""
+        self._lock.release()
+
+    async def __aenter__(self) -> None:
+        """Provides the asynchronous context manager syntax `async with ... :` when using the lock."""
+        await self._lock.acquire()
+        return None
+
+    async def __aexit__(self, exc_type, exc, tb) -> None:
+        """Provides the asynchronous context manager syntax `async with ... :` when using the lock."""
+        self._lock.release()