Graph trait for abstract nodes store

Graph trait with the method:

​find_nodes(node_type: NodeType, name: &str) -> Vec<Node>​

This we can use in many places. For example, instead of the find_only_one_function_file(func_name: &str, graph: &Graph) function in ast/src/lang/parse.rs, we can add another method to Graph, which uses self.find_nodes internally.

Sometimes its good to avoid a generic arg, like for find_function_parent. So try a closure arg instead, something like:

fn find_function_parent(&self, _cf: &dyn Fn(&str) -> Option<NodeData>) -> Result<Option<Operand>> {
        Ok(None)
}

Start with ast/src/lang/parse.rs, especially find_only_one_function_file, and trace the existing Graph, NodeType, Node, and NodeData abstractions. Determine the scope of the Graph trait and whether find_function_parent should use a closure; the work is done when the shared lookup methods and their callers use the agreed abstraction.