Significant tweak here - allowing "anonymous" nodes to cater for
voluminous, similar data such as time series.  Main change is in GetNode
method in doclayer.go (renamed from NewNode), which must now make 2nd
leg of indirection optional.  Anonymous doc is intuitive, but still not
sure if/how anonymous kind would be useful, oh well the functionality is
there now.  Existing test passes, but have not yet extended coverage.