Add an ast.Walk style iterator of GoStructs. (#890)

* Add an ast.Walk style iterator of GoStructs.

* ast.Walk: https://go.dev/src/go/ast/walk.go
* Rewrite ForEachField using the new `util.Walk` iterator, including supporting the PathQueryNodeMemo.
* Rewrite ForEachDataField and ForEachDataField2 using the new `util.Walk` iterator.

* [fix] import but not used yreflect in reflect.go

* [fix] go fmt walk.go

Author: shichuzhu