merge generate_children into do_iteration

Also fix this really weird slow down we get from doing direct mutation
to a state.allocator.get_ref (i.e. a Node by reference).  Maybe the
compiler just can't inline that instruction?  Regardless, this patch
fixes the slow downs!

src/worker.cpp

@@ -16,42 +16,40 @@ namespace cw::worker
   using cw::node::Fraction;
   using cw::node::Node;
 
-  std::tuple<u64, u64> generate_children(NodeAllocator &allocator, u64 index)
-  {
-    Node node = allocator.get_val(index);
-    if (node.left < 0)
-    {
-      allocator.get_ref(index).left = allocator.alloc(Fraction{
-          node.value.numerator, node.value.numerator + node.value.denominator});
-    }
-    if (node.right < 0)
-    {
-      allocator.get_ref(index).right = allocator.alloc(
-          Fraction{node.value.numerator + node.value.denominator,
-                   node.value.denominator});
-    }
-    return {node.left, node.right};
-  }
-
   void do_iteration(State &state)
   {
-    // state.mutex.lock();
+    state.mutex.lock();
     if (state.queue.empty())
     {
       // Unlock since there isn't any work to be done.
-      // state.mutex.unlock();
+      state.mutex.unlock();
       return;
     }
     u64 index = state.queue.front();
     state.queue.pop();
 
-    u64 left_child, right_child;
-    std::tie(left_child, right_child) =
-        generate_children(state.allocator, index);
+    Node node = state.allocator.get_val(index);
 
-    state.queue.push(left_child);
-    state.queue.push(right_child);
-    // state.mutex.unlock();
+    i64 left = node.left, right = node.right;
+    if (left < 0)
+    {
+      left = state.allocator.alloc(Fraction{
+          node.value.numerator, node.value.numerator + node.value.denominator});
+    }
+    if (right < 0)
+    {
+      right = state.allocator.alloc(
+          Fraction{node.value.numerator + node.value.denominator,
+                   node.value.denominator});
+    }
+
+    Node &node_ref = state.allocator.get_ref(index);
+    node_ref.left  = left;
+    node_ref.right = right;
+
+    state.queue.push(left);
+    state.queue.push(right);
+    state.mutex.unlock();
   }
 
   void worker(State &state)

src/worker.hpp

@@ -18,11 +18,7 @@ namespace cw::worker
   using cw::node::NodeAllocator;
   using cw::state::State;
   constexpr auto THREAD_PAUSE_DELAY   = std::chrono::milliseconds(1000);
-  constexpr auto THREAD_GENERAL_DELAY = std::chrono::nanoseconds(100);
-
-  // Given `index`, return the indices of its children in the tree.  If not
-  // present already, generate them using the allocator.
-  std::tuple<u64, u64> generate_children(NodeAllocator &allocator, u64 index);
+  constexpr auto THREAD_GENERAL_DELAY = std::chrono::milliseconds(10);
 
   // Performs a single iteration which consists of the following:
   // 1) pop an index off the iteration queue