Don't assume that iterators are just pointers.

That is the case for re2::StringPiece and absl::string_view
and happens to be the case for std::string_view in libstdc++
and libc++, but std::string_view in MSVC has iterators that
aren't just pointers.

Fixes #214.

Change-Id: I99542a1b53f138228a54f3845df140465a71980d
Reviewed-on: https://code-review.googlesource.com/c/re2/+/44230
Reviewed-by: Paul Wankadia <junyer@google.com>
11 files changed