The general assembly shall provide by general laws for the organization and classification of cities and towns. The number of such classes shall not exceed four; and the powers of each class shall be defined by general laws so that all such municipal corporations of the same class shall possess the same powers and be subject to the same restrictions. The general assembly shall also make provisions, by general law, whereby any city, town or village, existing by virtue of any special or local law, may elect to become subject to, and be governed by, the general laws relating to such corporations.