Add unused_format_specs lint

[Alexendoo]

Currently catches two cases:

An empty precision specifier:

// the same as {}
println!("{:.}", x);

And using formatting specs on format_args!():

// prints `x.`, not `x    .`
println("{:5}.", format_args!("x"));

changelog: new lint: [unused_format_specs]

[rust-highfive]

r? @xFrednet

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[bors]

☔ The latest upstream changes (presumably #9600) made this pull request unmergeable. Please resolve the merge conflicts.

[xFrednet]

Looks good to me, two small doc adjustments and one question. Then you can r=me 🙃

[xFrednet]

Thanks!

@bors r+

[bors]

📌 Commit 136c2cd has been approved by xFrednet

It is now in the queue for this repository.

[bors]

⌛ Testing commit 136c2cd with merge 502e87c...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: xFrednet
Pushing 502e87c to master...