[useless_vec]: lint on vec![_] invocations that adjust to a slice

[y21]

Fixes #2262 (well, actually my PR over at #10901 did do most of the stuff, but this PR implements the one last other case mentioned in the comments that my PR didn't fix)

Before this change, it would lint (&vec![1]).iter().sum::<i32>(), but not vec![1].iter().sum::<i32>(). This PR handles this case.
This also refactors a few things that I wanted to do in my other PR but forgot about.

changelog: [useless_vec]: lint on vec![_] invocations that adjust to a slice

[rustbot]

r? @Manishearth

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[Manishearth]

r? @Centri3 since you reviewed the first one

[rustbot]

Failed to set assignee to Centri3: invalid assignee

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[Manishearth]

@bors delegate=Centri3

[bors]

✌️ @Centri3, you can now approve this pull request!

If @Manishearth told you to "r=me" after making some further change, please make that change, then do @bors r=@Manishearth

[Centri3]

Will take a look in a bit :)

[Centri3]

This looks great, thanks! @Manishearth, can you take a quick look just in case I missed something?

[Manishearth]

@bors r=Centri3

thanks!

[bors]

📌 Commit 50ba459 has been approved by Centri3

It is now in the queue for this repository.

[bors]

⌛ Testing commit 50ba459 with merge da56c35...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Centri3
Pushing da56c35 to master...