resolve compilation issues in ast.hpp due to cyclic dep

[evaleev]

No description provided.

[Krzmbrzl]

Where was the cyclic dependence and how does this solve it? Tome the diff looks like your commit mainly makes the constructors explicit 🤔

[evaleev]

Where was the cyclic dependence and how does this solve it? Tome the diff looks like your commit mainly makes the constructors explicit 🤔

Definition NullaryValue was required to instantiate the Product ctor but NullaryValue was defined in terms of Sum so its definition was needed. Of course Sum also requires Product

[evaleev]

not sure why C++20 triggered the complaint. Indeed might be a compiler issue actually