Switch name Producer/Consumer -> Sender/Receiver

The producer/consumer was created for:
https://github.com/ArthurSonzogni/FTXUI/pull/11

This patch makes rename everything from Producer/Consumer toward
Sender/Receiver.

include/ftxui/component/receiver.hpp

@@ -0,0 +1,103 @@
+#ifndef FTXUI_COMPONENT_RECEIVER_HPP_
+#define FTXUI_COMPONENT_RECEIVER_HPP_
+
+#include <atomic>
+#include <condition_variable>
+#include <functional>
+#include <memory>
+#include <mutex>
+#include <queue>
+
+namespace ftxui {
+
+// Usage:
+//
+// Initialization:
+// ---------------
+//
+// auto receiver = MakeReceiver<std:string>();
+// auto sender_1= receiver.MakeSender();
+// auto sender_2 = receiver.MakeSender();
+//
+// Then move the senders elsewhere, potentially in a different thread.
+//
+// On the producer side:
+// ----------------------
+// [thread 1] sender_1->Send("hello");
+// [thread 2] sender_2->Send("world");
+//
+// On the consumer side:
+// ---------------------
+// char c;
+// while(receiver->Receive(&c)) // Return true as long as there is a producer.
+//   print(c)
+//
+// Receiver::Receive() returns true when there are no more senders.
+
+// clang-format off
+template<class T> class SenderImpl;
+template<class T> class ReceiverImpl;
+template<class T> using Sender = std::unique_ptr<SenderImpl<T>>;
+template<class T> using Receiver = std::unique_ptr<ReceiverImpl<T>>;
+template<class T> Receiver<T> MakeReceiver();
+// clang-format on
+
+// ---- Implementation part ----
+
+template <class T>
+class SenderImpl {
+ public:
+  void Send(T t) { sender_->Receive(std::move(t)); }
+  ~SenderImpl() { sender_->senders_--; }
+
+ private:
+  friend class ReceiverImpl<T>;
+  SenderImpl(ReceiverImpl<T>* consumer) : sender_(consumer) {}
+  ReceiverImpl<T>* sender_;
+};
+
+template <class T>
+class ReceiverImpl {
+ public:
+  Sender<T> MakeSender() {
+    senders_++;
+    return std::unique_ptr<SenderImpl<T>>(new SenderImpl<T>(this));
+  }
+
+  bool Receive(T* t) {
+    while (senders_) {
+      std::unique_lock<std::mutex> lock(mutex_);
+      while (queue_.empty())
+        notifier_.wait(lock);
+      if (queue_.empty())
+        continue;
+      *t = std::move(queue_.front());
+      queue_.pop();
+      return true;
+    }
+    return false;
+  }
+
+ private:
+  friend class SenderImpl<T>;
+
+  void Receive(T t) {
+    std::unique_lock<std::mutex> lock(mutex_);
+    queue_.push(std::move(t));
+    notifier_.notify_one();
+  }
+
+  std::mutex mutex_;
+  std::queue<T> queue_;
+  std::condition_variable notifier_;
+  std::atomic<int> senders_ = 0;
+};
+
+template <class T>
+Receiver<T> MakeReceiver() {
+  return std::make_unique<ReceiverImpl<T>>();
+}
+
+}  // namespace ftxui
+
+#endif  // FTXUI_COMPONENT_RECEIVER_HPP_