Suggest using `String::extend` where appropriate

What it does

The String type has some Extend implementations, and the lint suggests using those instead of pushing in a loop.

Advantage

• More concise
• I suspect it optimizes better

Drawbacks

No response

Example

for c in s.chars() {
    out_string.push(c);
}

Could be written as:

out_string.extend(s.chars());

And similar for e.g. String::push_str in a loop

Comparison with existing lints

The closest related lint I can find is string_extend_chars, which would replace the output for the example I provided with out_string.push_str(s);. For other Iterator<char>s (e.g. core::char::ToLowercase was the iterator I was using when inspired to make this issue ticket), the String::extend call would most likely be the last suggestion we can make.

Additional Context

It might be reasonable to extend to other data structures beyond just Strings.