Constexpr & template parameter for `split`

In this release:

• Iterators have become constexpr compatible if one uses C++20, because the iterators of dynamic containers such as std::vector::begin are only constexpr if C++20 is used (and the compiler has enough support for this).
• The function lz::split<SubString, std::string_view, char>(const std::string_view&, char delimiter) (previously lz::split<SubString, std::string_view, std::string>(const std::string_view&, char delimiter)) has been fixed. Its delimiter to search for was set wrong, it should've been char, but it was std::string causing a compilation error.
• Minor docs change.