expand_list to interactively work on a list of subgoals

also Tactical.VALID_LT, to check that a list-tactical is valid
(expand_list checks validity, expand_listf doesn't)

also abbreviations elt, eall, eta, enth to apply (list-)tactic to subgoal(s)
VALIDATE and VALIDATE_LT

If tac (ltac) is invalid due to proving theorems with extra assumptions,
VALIDATE tac and VALIDATE_LT ltac  make the tactic valid by returning extra
subgoals to prove the extra assumptions
Merge pull request #216 from jeremydaw/expand

expand_list to interactively work on a list of subgoals