The new gg format is based on a BNF file which can be found in the `gg`
directory. The code for decoding `.gg` files has been refactored to
mirror that file. The result is more resilient parsing, better errors,
and a greater ability to extend the format in the future.
The new decoder is notable in that it does not use a lexer. Both lexing
and parsing are done in a single step.
The format syntax itself has also been modified. Rather than using
semi-colons everywhere, commas are used as separators in tuples.
Additionally the final comma/semi-colon is no longer required.
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.
