[unnecessary_to_owned]: catch to_owned on byte slice to create temporary &str

[y21]

Closes #11648

Detects the pattern &String::from_utf8(bytes.to_vec()).unwrap() and suggests core::str::from_utf8(bytes).unwrap(), which avoids the unnecessary intermediate allocation.

I decided to put this in the existing unnecessary_to_owned lint (rather than creating a new lint) for a few reasons:

• we get to use some of its logic (for example, recognizing any of the functions in the to_owned family, e.g. to_vec)
• the actual inefficient operation that can be avoided here is the call to .to_vec(), so this is in a way similar to the other cases caught by unnecessary_to_owned, just through a bunch of type conversions
• we can make this more "generic" later and catch other cases, so imo it's best not to tie this lint specifically to the String type

changelog: [unnecessary_to_owned]: catch &String::from_utf8(bytes.to_vec()).unwrap() and suggest core::str::from_utf8(bytes).unwrap()

[rustbot]

r? @giraffate

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[bors]

☔ The latest upstream changes (presumably #11694) made this pull request unmergeable. Please resolve the merge conflicts.

[xFrednet]

(I claimed a few PRs, let's give this one to the lottery)

r? clippy