Introduce 'recur' callback

[ghost23]

Hi,

for my projects, I felt the need for an in-order callback during tree traversal.
(e.g. https://en.wikipedia.org/wiki/Tree_traversal -> in-order)

This pull request introduces a 'recur' callback beside 'enter' and 'leave'. recur is called with the parent element between the traversal of its children.

Think of this simple graph:

  Parent
    /\
   /  \
  A    B

With recur, the overall sequence of traversal will be: Enter Parent, Enter A, Leave A, Recur Parent, Enter B, Leave B, Leave Parent. This, beside other usecases, can be useful to check, wether a node is actually a grandchild or a sibling of another node, which can be hard to detect with enter and leave alone.

Recur here only supports BREAK. Also, within replace, recur does not support replacing or removing, only BREAK.

It is just a little addition, but perhaps you find it useful, too.

Cheers,
Sven

[Constellation]

I think https://github.com/estools/esrecurse is useful for this use case. How about this?

[ghost23]

I am not sure I understand, what node.left node.right are supposed to mean in esrecurse.

Anyway, I'd rather have the functionality in estraverse than having to use a different lib for special use cases.