feat add dispatcher to sled.h

#include "sled/synchronization/mutex.h"

#include <memory>

#include <set>

#include <type_traits>

namespace sled {

namespace experimental {

template<typename T>

class Dispatcher {

public:

    static_assert(!std::is_rvalue_reference<T>::value, "must be a value type");

    enum class DispatchResult { kHandled, kNotFound };

    struct Handler {

        virtual ~Handler()                    = default;

        virtual bool HandleMessage(const T &) = 0;

        virtual bool OnMessage(T &&)          = 0;

        virtual bool OnMessage(const T &)     = 0;

    };

    virtual ~Dispatcher() = default;

#include "inja.hpp"

#include "rx.h"

// experimental

#include "sled/experimental/design_patterns/dispatcher.h"

// event_bus

#include "sled/event_bus/event_bus.h"