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

[peterhuene]

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.