implement Trait Syn::Visit
## Example
rust
extern crate proc_macro2;
extern crate syn;
extern crate syn_query;
use proc_macro2::Span;
use syn::{ExprStruct, FieldValue, Ident};
use syn_query::Queryable;
fn main() {
let s = "Point { x: 1, y: 1 }";
let st: ExprStruct = syn::parse_str(s).unwrap();
let qr = st.query::<Ident>();
assert_eq!(qr[0].data, Ident::new("Point", Span::call_site()));
assert_eq!(qr[0].path, vec![0i64, 0i64, 0i64]);
assert_eq!(qr[1].data, Ident::new("x", Span::call_site()));
assert_eq!(qr[1].path, vec![2i64, 0i64, 0i64]);
assert_eq!(qr[2].data, Ident::new("y", Span::call_site()));
assert_eq!(qr[2].path, vec![3i64, 0i64, 0i64]);
let qr = st.query::<FieldValue>()
.filter(|x| x.path[0] == 3)
.query::<Ident>();
assert_eq!(qr[0].data, Ident::new("y", Span::call_site()));
assert_eq!(qr[0].path, vec![3i64, 0i64, 0i64]);
let qr = st.children::<syn::Path>()
.children::<syn::PathSegment>()
.children::<Ident>();
assert_eq!(qr[0].data, Ident::new("Point", Span::call_site()));
assert_eq!(qr[0].path, vec![0i64, 0i64, 0i64]);
}