An empty `Value` is now valid.
It is now possibly to change the edgeVal of an OpenEdge. It feels like
this shouldn't be necessary, but it greatly simplifies the decoding
logic to have this.
A tuple which is created with just one input edge, and with no edge
value of its own, is now automatically simplified to just that input
edge.
For MVP newlines aren't going to be used as a syntax terminator, they're
just going to be whitespace. Otherwise the decoding logic gets way more
complicated.
`gg.Graph` has been reworked in its internal functionality, to more
closely match the capability of a purely stack-based implementation. The
current implementation is _very_ inefficient, however. Tests have been
deliberately left pretty sparse, as I expect the internals to continue
to change significantly.
