Implement Display trait for string::Match

[volsa]

I regularly have to convert matches of a named capture group into a string, which with the current master branch is achieved by capture.as_str().to_string(). This PR implements the Display trait to simplify the given snippet into capture.to_string().

[BurntSushi]

Display is for user facing output, and there are many ways in which a Match could be formatted for users. Because of that, this trait impl isn't appropriate.

[volsa]

Thanks for the feedback! I'm just curious and would like to understand why a user would want to see anything other than the haystack field? Put differently there are only two other fields (start, end) which I reckon are only interesting for debug cases and already covered by the Debug trait hence wouldn't the Display trait be valid for just displaying haystack?

Edit: Is it because of e.g. user facing output like "Haystack: '...'"?

[BurntSushi]

Offsets might be important to users. Grep for example has options to print offsets. Offsets aren't just for debugging.