reimplement Row using sorted lists of gaps
reimplement Row using sorted lists of gaps

instead of storing a row as a string,
now two arrays are stored:

 - list of block positions of starts of groups of nongaps
 - list of number of nongaps before corresponding
    groups in first list

npge/lua-npge,starius/lua-npge,starius/lua-npge,npge/lua-npge,starius/lua-npge,npge/lua-npge