Instead of opting in std::string_view support via PUGIXML_STRING_VIEW
define, always enable it when C++17 is supported; this still requires
enabling C++17 support in the compiler, which this change doesn't
attempt to do yet.
When PUGIXML_STRING_VIEW define is set and C++17 is available, add std::string_view support to a few functions. In the future, string view support will be enabled without the need for an extra define, but for now the support is opt-in to reduce compatibility risks.
PR is based on initial contribution by @brandl-muc.
