Added a constructor for AstNode

[sunjay]

Hello again!
I added a constructor for AstNode so we don't have to go through 3 constructors every time just to create one.

The issue I'm addressing is that AstNode is Node<'a, RefCell<Ast>> which is three nested objects. That means to make one from a NodeValue, you have to use three constructors:

Node::new(RefCell::new(Ast::new(value)))

Rather than just make a utility function in my code, I thought it would be better to just add a constructor so you can do AstNode::from(value) instead.

If/when this gets merged, could you release a new patch version of comrak? Thanks as always! 😄 🎉

Documentation Screenshot

[kivikakk]

👌 Thank you!

[kivikakk]

Done -- https://github.com/kivikakk/comrak/releases/tag/0.4.4.

[sunjay]

Thank you so much! 😊

[kivikakk]

Just FYI (and I'm not sure this'll be relevant given how much time has passed), I'm planning to remove this From impl in 0.18 — with proper line/col tracking to come, I'm discouraging creating Asts with default (zero) values for these fields.