This change required OpenEdges to be passed around as pointers, which in
turn required me to audit how value copying is being done everywhere and
simplify it in a few places. I think I covered all the bases.
The new internals of Graph allow the graph's actual structure to be
reflected within the graph itself. For example, if the OpenEdges of two
ValueIns are equivalent then the same OpenEdge pointer is shared for the
two internally. This applies recursively, so if two OpenEdge tuples
share an inner OpenEdge, they will share the same pointer.
This change is a preliminary requirement for creating a graph mapping
operation. Without OpenEdge deduplication the map operation would end up
operating on the same OpenEdge multiple times, which would be incorrect.
The base graph implementation has been moved into its own package,
`graph`, and been made fully generic, ie the value on each vertex/edge
is a parameterized type. This will allow us to use the graph for both
syntax parsing (gg) and runtime evaluation (vm), with each use-case
being able to use slightly different Value types.
