Implements analogue of implication-left sequent calculus rule as tactic
Given a goal of the form A ?- ((p ==> q) ==> r), an application of
impl_tac will produce two sub-goals: A ?- p and A ?- (q ==> r).
This can be useful if p should be dealt with in isolation, when,
say, the tactics that solve p can’t safely be applied to q and/or
Fails if the goal is not an implication with another implication as
its antecdent. Note that for the purpose of this tactic, a negation
~p is viewed as the implication p ==> F. This means that
impl_tac will succeed when applied to goals whose conclusions are
~p ==> q, ~(p ==> q) and ~~p.