string_path - std::basic_string_view enhancements

[mknaleczb]

In case of using C++17 standard or higher:

1. string_path<> class gets new typedef strview_type = std::basic_string_view<char_type>;
2. string_path<>::string_path(const String&, ...) is replaced by string_path<>::string_path(strview_type, ...);
3. dump_sequence(...) functions are modified from const {std::string/std::wstring}& to {std::string_view/std::wstring_view}

Reasoning:
During using boost::property_tree::wptree with standard C++20 I needed to do some special actions when using std::wstring_view due to std::basic_string(std::basic_string_view) ctor being explicit:

std::wstring_view pTreePath{L"some.example"};
wpath rootPTreePath = wpath(L"root") / std::wstring(pTreePath);
wptree pTree{};
...
const auto val = pTree.get_optional(pTreePath.data());

---

For all standards:

1. Added string_path<>::to_str() to expose string_path<>.m_value

Reasoning:
As someone works with wpath and with std::wstring, string_path<>::dump() forces conversion at first from (String == std::wstring) --(dump())-> std::string --(some other method)-> std::wstring