Auto merge of #11656 - y21:unnecessary_string_from_utf8, r=Jarcho

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

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()`

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