Provide a shortcut function to get Name in Semantic mode

Having obtained function names in #365 I need to get the actual strings. My current take is

```go
nodeName, err := tools.FilterNode(node, "//uast:Function/../../Name")
if err != nil {
	return nil, err
}
println(string(nodeName.(nodes.Object)["Name"].(nodes.String)))
```

I have to hardcode `"Name"` and do two type assertions. Is it possible to add a wrapper?