go-merkledag's ProtoNode may be modified such that common method calls may panic

QFinch assessment

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.

Potential impact

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-formatBlock and github.com/ipfs/go-ipld-formatNode interfaces, certain methods, which internally require a re-encode if state has changed, will panic due to the inability to return an error. Additionally, use of the ProtoNodeSetCidBuilder() method to set a non-functioning CidBuilder (such as one that refers to a multihash where an implementation of that hash function is not available) may cause the same methods to panic as a new CID is required but cannot be created.