Deduplicate parts of code between basic_string and basic_string_view

[qookei]

Certain basic_string operations (such as comparisons) can be implemented in terms of basic_string_view: e.g.:

bool operator==(const basic_string &other) const {
    return basic_string_view<Char>{_buffer, _length} == basic_string_view<Char>{other._buffer, other._length};
}

We could also add a convenience basic_string::view() method that does the view construction.

Doing this will allow us to more easily prevent bugs like the one fixed by 264af2d.