Get rid of `using std::string;`. (part 3 of N)

Change-Id: I24c9a04ba92ec6503bb040658f07e7aabacf74f2
Reviewed-on: https://code-review.googlesource.com/c/re2/+/39114
Reviewed-by: Paul Wankadia <junyer@google.com>
9 files changed