President Enrique Pena Nieto signed into law on Monday new rules governing a historic opening of Mexico's state-run oil, gas and electricity industries to foreign and private companies. Pena Nieto said the government will let potential investors know by Wednesday which blocks of gas and oil fields will be open for them. The state-owned oil company, Petroleos Mexicanos, or Pemex, has the right under the new rules to take first dibs and set aside s…