Use StringPieces for RE2::Scanner matches

Rather than allocating new strings when consuming matches for
`RE2::Scanner#scan`, use `StringPiece`s (actually `absl::string_view`s
in newer versions of RE2) to refer to the original input string instead.

ext/re2/re2.cc

@@ -359,7 +359,7 @@ static VALUE re2_scanner_scan(VALUE self) {
 
   std::vector<RE2::Arg> argv(c->number_of_capturing_groups);
   std::vector<RE2::Arg*> args(c->number_of_capturing_groups);
-  std::vector<std::string> matches(c->number_of_capturing_groups);
+  std::vector<re2::StringPiece> matches(c->number_of_capturing_groups);
 
   if (c->eof) {
     return Qnil;