Wrapping up 'order' and 'setorder'

+ All instances of 'forder' renamed to 'forderv': naming consistency
+ 'fast_order' renamed to 'forder' : naming consistency
+ 'DT[order(x, -y, …)]' automatically optimised to 'DT[forder(DT, x, -y, …)]'.
+ Test added for above change.
+ Updated news with entry for '?setorder' and 'DT[order(…)]' optimisation