Get rid of pre and post traverses. Make one with pre and post callbacks

[danyfedorov]

// for example 
function pTrav(iTree, { preCallback, postCallback, childrenResolutionType }) { ... }

Now I want to get rid of the depthFirst dir and just have:

travDF
rewriteDF
travBF
rewriteBF
(maybe add searchDF, searchBF ?)
iTreeToVertex
collect