add node iteration apis
add node iteration apis

TODO: add tests

Akasurde/pytest,davidszotten/pytest,nicoddemus/pytest,ddboline/pytest,nicoddemus/pytest,alfredodeza/pytest,hackebrot/pytest,txomon/pytest,pfctdayelise/pytest,RonnyPfannschmidt/pytest,The-Compiler/pytest,The-Compiler/pytest,tomviner/pytest,markshao/pytest,pytest-dev/pytest,tomviner/pytest