Consider adding a depth limit and/or caching to Array::min(), Array::max(), etc

[arcondello]

Right now calling ::max() on an array symbol can trigger a dfs. It would be usful to have some way of improving that. Either memoizing, e.g.

    virtual double min(std::unordered_map<const Array*, double> memo) const {
        auto it = memo.find(this);
        if (it != memo.end()) return it->second;
        auto min_ = this->min();
        memo.emplace_hint(it, this, min_);
        return min_;
    }

or setting a depth limit

    virtual double min(ssize_t depth = -2) const {
        if (depth == -1) return inf;
        ...
    }

or both or something else.