Fix cpp2::to_string to call cpp2::to_string instead of std::to_string

[filipsajdak]

Specialisation of cpp2::to_string function for std::optional is using std::to_string on optional value.

template<typename T>
auto to_string(std::optional<T> const& o) -> std::string {
    if (o.has_value()) {
        return std::to_string(o.value());
    }
    return "(empty)";
}

That makes the below code fail to compile:

std::optional<std::string> v { "this will not work" }; 
std::court << "(v)$" << std::endl;

cppfront will generate

std::optional<std::string> v { "this will not work" }; 
std::cout << "My map: " + cpp2::to_string(v) + "\n" << std::endl;

The result is that there is a call to std::to_string(std::string) and there is no such specialization in the standard library.

Closes #38

[filipsajdak]

I have added examples of use in regression tests and results of execution.