Lint against calling to_string() on &&str, &&String, etc.

[sinkuu]

ToString is specialized for string types to avoid using expensive formatting facility. This specialization can be missed by calling to_string on redundantly-referenced strings like &&str or &&String which may easily occur when working with iterators, for example:

// Generic implementation for `T: Display` is used (slow)
["foo", "bar"].iter().map(|s| s.to_string());

// OK, the specialized impl is used
["foo", "bar"].iter().map(|&s| s.to_string());

[HMPerson1]

I can work on this

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[mati865]

Oh #4595 wasn't merged.