walk a directed acyclic graph, given a key function, comparison function, find function, and a next function.
var walk = dag(['HASH1', 'HASH2'], key, find, cmp, next)
walk(function(err, data) {
// data === undefined if no more items
})
create a dag walker using the following definitions:
key_function(object) -> "key"find_function("key", ready(err, data))comparison_function(object, object) -> [-1...1]next_function(object) -> ["key", "key", ...]MIT