Reduce and prevent unnecessary random-access to List

Random-access access to `List` when iterating is `O(n^2)` (`O(n)` when
accessing a single element)

* Removed subscript operator, in favor of a more explicit `get`
* Added conversion from `Iterator` to `ConstIterator`
* Remade existing operations into other solutions when applicable

core/debugger/remote_debugger_peer.cpp

@@ -45,7 +45,7 @@ bool RemoteDebuggerPeerTCP::has_message() {
 Array RemoteDebuggerPeerTCP::get_message() {
 	MutexLock lock(mutex);
 	ERR_FAIL_COND_V(!has_message(), Array());
-	Array out = in_queue[0];
+	Array out = in_queue.front()->get();
 	in_queue.pop_front();
 	return out;
 }
@@ -100,7 +100,7 @@ void RemoteDebuggerPeerTCP::_write_out() {
 				break; // Nothing left to send
 			}
 			mutex.lock();
-			Variant var = out_queue[0];
+			Variant var = out_queue.front()->get();
 			out_queue.pop_front();
 			mutex.unlock();
 			int size = 0;