list_tactic concept

introduce list_tactic - works on a list of goals, not a single goal
THEN_LT (tac1, ltac2 : list_tactic),
  A tactical that applies ltac2 to the list of subgoals resulting from tac1
TACS_TO_LT (tac2l: tactic list) : list_tactic
ALLGOALS - A list_tactic which applies a given tactic to all goals
ALL_LT - always succeeds
NTH_GOAL - applies tactic to nth goal of list
REVERSE_LT -  A list_tactic that reverses a list of subgoals
SPLIT_LT - apply two list-tactics, one to first n subgoals,
  the other to the rest
ROTATE_LT - rotate the list of subgoals by n positions
Merge branch 'trial' into tmp
Tactical.VALID_LT

Tactical.VALID_LT to check that a list-tactical is valid,
expand_list to interactively apply a list_tactic and check its validity;
abbreviations elt, eall to apply tactic to all subgoals