What do the powers that be in midtown Manhattan do when their foot-weary citizens need a place to sit down in the middle of a street? Why they turn to concrete, of course. Tired walkers striding along Broadway can soon rest their tired feet along a two-block stretch where the Fashion Center Business Improvement District has unveiled 20 cast-concrete furniture pieces. The chairs and tables are simple cubic and cylindrical pieces. When they're rolled again later this month, they will be placed atop a repainted plaza between West 39th and West 40th streets through April.