tools.target: use slots for Channel and User types

This can save some significant memory when Sopel is in many and/or large
channels. For example, without this patch (in bytes):

- Size of empty channel: 1200
- Size of channel with 100 users: 84560
- Size of channel with 1000 users: 817944

And with this patch (also in bytes):

- Size of empty channel: 680
- Size of channel with 100 users: 72608
- Size of channel with 1000 users: 705304

An empty channel is reduced by nearly half, and channels with some
significant user count can be reduced by some 10-15%.

These values were calculated using `Pympler.asizeof.asizeof()`, in a
simple test script:

    from sopel.tools import Identifier, target
    from pympler.asizeof import asizeof

    c = target.Channel(Identifier('#Sopel'))

    print('Size of empty channel:', asizeof(c))

    for n in range(1, 100):
        name = 'user{}'.format(n)
        c.add_user(
            target.User(
                Identifier(name)
                name,
                'some.example.org'
            )
        )

    print('Size of channel with 100 users:', asizeof(c))

    for n in range(101, 1000):
        name = 'user{}'.format(n)
        c.add_user(
            target.User(
                Identifier(name),
                name,
                'some.example.org'
            )
        )

    print('Size of channel with 1000 users:', asizeof(c))

sopel/tools/target.py

@@ -15,6 +15,10 @@ class User:
     :param str user: the user's local username ("user" in `user@host.name`)
     :param str host: the user's hostname ("host.name" in `user@host.name`)
     """
+    __slots__ = (
+        'nick', 'user', 'host', 'channels', 'account', 'away',
+    )
+
     def __init__(self, nick, user, host):
         assert isinstance(nick, Identifier)
         self.nick = nick
@@ -59,6 +63,10 @@ class Channel:
     :param name: the channel name
     :type name: :class:`~.tools.Identifier`
     """
+    __slots__ = (
+        'name', 'users', 'privileges', 'topic', 'modes', 'last_who', 'join_time',
+    )
+
     def __init__(self, name):
         assert isinstance(name, Identifier)
         self.name = name