Old idea: Add `.isAst()`

[tomek-he-him]

ast.isAst(object) should return true when the object ist a valid AST.

[tomek-he-him]

If we go with #22, this won’t be necessary anymore.

The current behavior (the one ast.isAst(object) would be helpful with) – is inspired by architectures like https://github.com/Matt-Esch/virtual-dom/tree/903d884a8e4f05f303ec6f2b920a3b5237cf8b92/vnode.