Implement DAG topology iterators

[coderofstuff]

Is your feature request related to a problem? Please describe.
Iterating forwards or backwards in topological DAG order from a root to some set of tips is a common operation in several parts of the code. It is currently commonly implemented as a BFS.

Describe the solution you'd like
To make this process less error prone, it is good to have an actual iterator for this operation.

[freshair18]

Noting that this functionality was at least partially implemented in #725.

Though IIRC I ended up not really using it (it was mainly meant for proof of publication functionality )