Add handling of composition of standard types

Current implementation does not allow to:
* put variant, pair, tuple inside optional,
* put pair, tuple inside variant,
* put pair inside tuple

This is just because order of definitions of to_string specialisations
matters. When compiler matches to_string for optional it is not
aware of existence of specialisations for variant, pair, tuple.

This change propose to add forward declarations to to_string
specialisations for variant, pair and tuple.

Author: filipsajdak

include/cpp2util.h

@@ -771,6 +771,15 @@ inline auto to_string(T const& sv) -> std::string
     return std::string{sv};
 }
 
+template <typename... Ts>
+inline auto to_string(std::variant<Ts...> const& v) -> std::string;
+
+template < typename T, typename U>
+inline auto to_string(std::pair<T,U> const& p) -> std::string;
+
+template < typename... Ts>
+inline auto to_string(std::tuple<Ts...> const& t) -> std::string;
+
 template<typename T>
 inline auto to_string(std::optional<T> const& o) -> std::string {
     if (o.has_value()) {