By Ayenat Mersie and Clement Uwiringiyimana KIGALI (Reuters) -Britain’s Prince Charles expressed deep sorrow over slavery in a speech to Commonwealth leaders in Rwanda on Friday and acknowledged that the roots of the organisation lay in a painful period of history. The Commonwealth, a club of 54 countries that evolved from the British Empire, encompasses about a third of humanity and presents itself as a network of equal partners, but some member states have been calling for a reckoning with the colonial past. “I want to acknowledge that the roots of our contemporary association run deep into …