Initial impl of unnecessary_first_then_check

[lukaslueg]

Fixes #11212

Checks for {slice/vec/Box<[]>}.first().is_some() and suggests replacing the unnecessary Option-construct with a direct {slice/...}.is_empty(). Other lints guide constructs like if let Some(_) = v.get(0) into this, which end up as !v.is_empty().

changelog: [unnecessary_first_then_check]: Initial implementation

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[Manishearth]

@bors r+

[bors]

📌 Commit 290a01e has been approved by Manishearth

It is now in the queue for this repository.

[bors]

⌛ Testing commit 290a01e with merge 9be28b1...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Manishearth
Pushing 9be28b1 to master...