Better thread pool constructor

statically define a number of threads, then setup the necessary
machinery to make it work.

src/main.cpp

@@ -116,13 +116,13 @@ int main(void)
   std::string format_str;
   u64 format_str_width = 0;
 
-  // Init threads
-  std::thread threads[] = {
-      std::thread(cw::worker::worker, std::ref(state)),
-      std::thread(cw::worker::worker, std::ref(state)),
-      std::thread(cw::worker::worker, std::ref(state)),
-      std::thread(cw::worker::worker, std::ref(state)),
-  };
+// Init threads
+#define THREADS 15
+  std::thread threads[THREADS];
+  for (auto i = 0; i < THREADS; ++i)
+  {
+    threads[i] = std::move(std::thread(cw::worker::worker, std::ref(state)));
+  }
 
   // Setup raylib window
   InitWindow(WIDTH, HEIGHT, "Calkin-Wilf tree");