Add lint for 'toilet closures'

[GnomedDev]

Closes #2523

This name is a little silly, but is also a widely used term and I don't think there is a funnier name we could use.

I feel like the discussion in that issue thread has come to a settle, with most people preferring drop to |_| () or suggesting those do not to implement their own fn ignore<T>(_: T) {} which would not fire this lint. It is also always possible to disagree with the agreed style of clippy with a simply allow attribute.

changelog: [toilet_closures]: Implemented lint for replacing |_| () with drop

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[blyxyas]

Hi! I made a poll on Zulip as I'm not sure how to feel about this lint

We'll see what the team thinks =^w^=

[Manishearth]

Closing based on poll https://rust-lang.zulipchat.com/#narrow/stream/257328-clippy/topic/.E2.9C.94.20Toilet.20Closures