This module makes it easy to visualise data structures, even recursive or
circular ones.

It is provided as an alternative to GraphViz::Data::Grapher. Differences:
   - GraphViz::Data::Structure handles structures of arbitrary depth and
     complexity, automatically following links using a standard graph
     traversal algorithm.
   - GraphViz::Data::Grapher creates graphics of indiividual substructures
     (arrays, scalars, hashes) which keep the substructure type and data
     together; GraphViz::Data::Structure does this by shape alone.
   - GraphViz::Data::Structure encapsulates object info (if any) directly
     into the node being used to represent the class.
   - GraphViz::Data::Grapher colors its graphs; GraphViz::Data::Structure
     doesn't by default.
   - GraphViz::Data:Structure can parse out globs and CODE references (almost
     as well as the debugger does).