Lint for needless use of `Cow`

What it does

When a Cow's Borrowed variant is only ever used for empty strings, clippy should suggest to remove the Cow and use String::new() instead.

Advantage

• String::new() allocates no memory, and is essentially the same speed: https://play.rust-lang.org/?version=nightly&mode=release&edition=2021&gist=93a8649afa1b55fc84cef2829920a932 (Less instructions doesn't mean faster, but imo, even if it's slower it's worth the ergonomics)
• It's simpler.

Drawbacks

• It may actually end up being ever so slightly slower. In some cases this may be undesirable.

Example

if let Some(x) = x {
    x.to_string().into()
} else if let Some(y) = y {
    Cow::Borrowed("")
} else {
    "".into()
}

Could be written as:

if let Some(x) = x {
    x.to_string()
} else if let Some(y) = y {
    String::new()
} else {
    String::new()
}

The issue names no files or tests. Start by searching rust-lang/rust-clippy for existing Cow-related lints and their tests, then compare the proposed examples with current lint behavior. Done means the described needless Cow cases are recognized and the suggested String::new() replacements are covered by regression tests.