Feature request: absl::wstring_view

As a Windows user, I use `std::wstring` to manipulate Windows file path. I do like to use `{std,absl}::wstring_view` for efficiency.

`absl/strings/string_view` currently implements `absl::string_view` class directly rather than implementing `absl::basic_string_view` then `using string_view = basic_string_view`.

The addition of `absl::u16string_view` and `absl::u32string_view` would be nice too, but I don't need them yet.

If Abseil team fear that people might do silly thing with `basic_string_view`, then just put it in `internal` namespace, I doubt if anyone actually wants to use `basic_string_view` directly.