dag: add prune support to FinderNode
types,dag: add support for attribute lookup on values

So far, strings and lists have no attributes, so lookup always fails
-- would have to change FuString and FuList to structs for that, which
will be a bit of upheaval.

All Node types support attribute lookup, though -- but there are no
attributes defined yet.
dag: export NewFileNode() instead of newFileNode()
types,runtime: add support for finding and calling methods