This was written because I originally intended to make the fields of the ast node types entirely private, but that made constructing them tedious so they are pub(crate) which coincidentally also allows them to be used by the iterator.
Ultimately this is about semver and exposing a stable interface while allowing the internal representation to change. The fields are still pub(crate) to make constructing the types easier inside this crate, which should be fine because we can refactor the code inside this crate whenever the internal structure changes.
This implementation will reduce the use of heap by elimininating Box<> from the individual iterators but it will still need heap for maintaining a vector of iterators from nodes.
