Implement `wac_graph::CompositionGraph::decode` to decode previous encodings

Tracking issue for decoding wasm files from a previous graph encoding back to a `CompositionGraph`, allowing `wac-parser` to be able to emit a WAC source representation of the graph.

As local names will require a `names` custom section in the resulting component, the decoding should manufacture local names when the `names` section is not present.