v0.0.13: Code and test updates

Code changes (in package order):
- Data::Hopen::hnew(): provide more accurate error messages.
- Data::Hopen::loadfrom(): no longer require a list of stems.
- Data::Hopen::G::DAG: pass the node inputs to visit_{node,goal}; change
  some exception text
- Data::Hopen::G::Entity: custom stringification for easier debugging
- Added Data::Hopen::G::NoOp

- !!! >>> Data::Hopen::Scope: when merging predecessor-node outputs
  into successor-node inputs, DO NOT CLONE.  This way blessed references
  can be successfully passed between nodes. <<< !!!

- Data::Hopen::Util::NameSet: Tweaked code so 100% coverage
  is achievable.
- Added Data::Hopen::Visitor
- Updated documentation

Test changes:
- Added t/lib/PackagesInThisFile, which is a crazy way of doing
  subtests, each in its own package.

WIP: a way to merge hashes, cloning non-blessed references but not
     references.