tools.target: use slots for Channel and User types

[dgw]

Description

Tin, really. Python has this feature where you can tell it to store instance attributes in a "slot", and using it can make small objects more time- and space-efficient by eliminating the need to look up attributes in the instance's __dict__.

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%.

I have not checked the impact of this patch on execution time, but would be glad to do so if requested.

Notes

These values were calculated using Pympler.asizeof (basically a recursive version of sys.getsizeof) 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))

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
  • flake8 flags some errors in my target_sizes.py test script, which I didn't bother to make PEP8-compliant.
• I have tested the functionality of the things this change touches

[Exirel]

Nitpicking of the year.

[Exirel]

Let's merge that asap.