#51 Better node system

• more compile-time checks
• better syntax
• better runtime performance

Something like this:

class Add : public TypedNode<double, Only<double>, Only<double>> {
    NODE_PROPERTIES(a, b)
    NODE_PROPERTY(a, 0)
    NODE_PROPERTY(b, 0)
public:
    double get(shared_ptr<Context> ctx) const {
        return a(ctx)+b(ctx);
    }
};

Better yet:

template <class Self, typename... Args>
class LinkStorage {
private:
    // guaranteed to be unique per Self
    static map<string,size_t>& name_map() {
        map<string,size_t> _instance;
        return _instance;
    }
public:
    static void register_name(string const& name) {
        ...
    }
};

template <class Self, typename Result, typename... Args>
class Node;

class Add : NODE(Add, double, Only<double>, Only<double>) {
    NODE_PROPERTIES(a, b) // using variadic macro expansion

public:
    double get(shared_ptr<Context> ctx) const override {
        return get_a(ctx) + get_b(ctx);
    }
};

Basic system is in place, though some enhancements to API are still expected.