Fixing old in triggers

[marcoeilers]

This PR fixes two issues:

In a quantifier with trigger f(e), it could happen that if the quantifier body contained old(f(e)), that Silicon accidentally emitted a trigger essentially equivalent to old(f(e)) instead of the intended trigger (that's the problem behind issue #509). The reason is that when remembering what terms expressions evaluate to, Silicon did not take into acount old, so when evaluating old(f(e)) to some term t during the evaluation of the quantifier body, it would incorrectly remember that f(e) evaluates to t, and thus translate a trigger f(e) accordingly. Now, Silicon correctly remembers that t is the value of old(f(e)).

Second, Silicon used to discard old on the outside of trigger expressions. So if the user supplied a trigger old(e), Silicon would treat this as if they'd just written e. Now it no longer does this.