Cleanup ast/*

[lestrrat]

I want to cleanup ast/* . For one thing I want to work towards squeezing more performance out of the library, but for that I want to centralize object value assignment and allocation. I'd like to apply this to a wider area than ast/, but I'd like to start at ast/

For now, I'd like to disallow explicit instantiation of, and assignment to structs. i.e.

// disallow 
n := ast.Foo{Start: tk, Value: blah}
n.Start = ...
n.Value = ...

// enforce encapsulation
n := ast.Foo(tk, blah)
n.Start() // Get only
n.SetValue(...) // If mutation is required

Would you consider this type of change if I submit a PR?