the beetle is the only climber
a simpler check for connected hive in `add`

When we implement moving a bug, then we'll need to use the graph
to check for a connected hive.  When adding a bug, it's enough
to check for neighbors.
remove `Hex` class

the `Hex` was unable to answer all but the most simple questions,
because it only knew about itself and its own bug.
comment about neighbors
extract method `legal_addition?`