Use flat-tree interator

[yoshuawuyts]

Feature Request

Summary

tree-index is currently the only module using flat-tree's with_depth_* range of methods. We could refactor this module to use the (stateful) flat-tree iterator in order to get rid of exposing the methods.

Motivation

Flat tree's with_depth methods are hard to get right, and the less API surface we can expose the better it is.

Expected Behavior

Replace this with iterator code: https://github.com/datrs/tree-index/blob/master/src/lib.rs#L185-L210.

Drawbacks

It's a refactor, and it might take a bit of time to get right. Maybe we should work on more tests first?

Rationale and alternatives

The code is pretty magicky as is; we can do better.

Unresolved Questions

None.