Prefer `.cloned()` over `.map(|foo| foo.clone())`

[pierrechevalier83]

There are other combinations of map and clone in the code, but
this commit only addresses the straightforward ones.

Fixes #7906.

[pierrechevalier83]

As mentioned in the commit message, I ignored a number of "kinda similar instances".
namely:

$ git grep -E "\.map\(\|.*\| .*\.clone\(\)\)"
components/gfx/platform/macos/font_template.rs:        ctfont.as_ref().map(|ctfont| (*ctfont).clone())
components/script/dom/eventtarget.rs:            listeners.iter().map(|entry| entry.listener.clone()).collect()
components/script/dom/eventtarget.rs:            filtered.map(|entry| entry.listener.clone()).collect()
components/script/dom/formdata.rs:        let name = filename.unwrap_or(f.map(|inner| inner.name().clone()).unwrap_or("blob".to_owned()));
components/script/dom/htmlfontelement.rs:                            .map(|value| value.as_atom().clone())
components/script/dom/htmlinputelement.rs:            .map(|name| name.value().as_atom().clone())
components/script/dom/xmlhttprequest.rs:                headers.as_ref().map(|h| *self.response_headers.borrow_mut() = h.clone());
components/util/prefs.rs:    PREFS.lock().unwrap().get(name).map(|x| x.value().clone()).unwrap_or(Arc::new(PrefValue::Missing))

Please, fail the review if you think some of these can and should also use .cloned()

[jdm]

Your choices look reasonable to me!

[jdm]

@bors-servo: r+

[bors-servo]

📌 Commit 7286a3d has been approved by jdm

[jdm]

@bors-servo: r-
Thanks for the effort @pierrechevalier83 :)

[pierrechevalier83]

thanks for bearing with me...