Get rid of `using std::string;`. (part 5 of N)

Change-Id: Icf7ef53354fce3c7d1f19d35799437e4b6f4f7fb
Reviewed-on: https://code-review.googlesource.com/c/re2/+/39132
Reviewed-by: Paul Wankadia <junyer@google.com>
7 files changed