some refactoring with optional_function (#96)

- wrapped std::function into utils::optional_function 
  - cleaned the code accordingly in windows_emulator
- using the 'emulator'/'windows_emulator' dependency implies the
emulator_common as well.

src/windows-emulator/syscalls.cpp

@@ -2681,7 +2681,7 @@ namespace
                 temp_buffer.push_back('\n');
             }
 
-            c.win_emu.on_stdout(temp_buffer);
+            c.win_emu.callbacks().stdout_callback(temp_buffer);
             c.win_emu.log.info("%.*s", static_cast<int>(temp_buffer.size()), temp_buffer.data());
 
             return STATUS_SUCCESS;