Provide list iterator interface

[hannahhoward]

Goals

Allows a UnixFS dirs and HAMT dirs to be interated as a list. This works the same as the MapIterator interface but allows accessing the TSize property.

Note when iterating a HAMT, only leaf nodes are returned, and the Name field is shortened to not include the hamt prefix

[willscott]

I wonder if we want to do this as a list -

The IPLD node interface for a directory will be of kind Map, so by the IPLD contract i would expect that asking for a list iterator returns `nil.

A non-ipld interface + method might be clearer for accessing this functionality.

[willscott]

I'm proposing that #60 is a potentially nicer way to expose the functionality that this.

this is mis-using the ipld interface a bit more, and while i don't expect problems from it, i'm a little sketched-out by the overloading.

[rvagg]

I think this is covered by #60 now