ProtoNode may be modified such that common method calls may panic in ipfs/go-merkledag

go-merkledag implements the 'DAGService' interface and adds two ipld node types, Protobuf and Raw for the ipfs project. A ProtoNode may be modified in such a way as to cause various encode errors which will trigger a panic on common method calls that don't allow for error returns. A ProtoNode should only be able to encode to valid DAG-PB, attempting to encode invalid DAG-PB forms will result in an error from the codec. Manipulation of an existing (newly created or decoded) ProtoNode using the modifier methods did not account for certain states that would place the ProtoNode into an unencodeable form. Due to conformance with the `github.com/ipfs/go-block-format#Block` and `github.com/ipfs/go-ipld-format#Node` interfaces, certain methods, which internally require a re-encode if state has changed, will panic due to the inability to return an error. This issue has been addressed across a number of pull requests. Users are advised to upgrade to version 0.8.1 for a complete set of fixes. Users unable to upgrade may attempt to mitigate this issue by sanitising inputs when allowing user-input to set a new CidBuilder on a ProtoNode and by sanitising Tsize (Link#Size) values such that they are a reasonable byte-size for sub-DAGs where derived from user-input.