Prevent letification from shadowing variables (#3042)

Prevent letification from shadowing variables (#3042)

Fixes #3005. When printing nodes, we introduce `let` expressions on the
fly. However, when doing that, we have to be careful that we don't
shadow existing variables with the same name. When quantifiers are
involved, we do not descend into the quantifiers to avoid letifying
terms with bound variables that then go out of scope (see #1863). Thus,
to avoid shadowing variables appearing in quantifiers, we have to
collect all the variables appearing in that term to make sure that the
let does not shadow them. In #3005, the issue was caused by a `let` that
was introduced outside of a quantifier and then was shadowed in the body
of the quantifier by another `let` introduced for that body.