Return all legal edge coordinates on the grid.

def legal_edge_coords():
    """
    Return all legal edge coordinates on the grid.
    """
    edges = set()
    for tile_id in legal_tile_ids():
        for edge in edges_touching_tile(tile_id):
            edges.add(edge)
    logging.debug('Legal edge coords({})={}'.format(len(edges), edges))
    return edges