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Per https://github.com/ipld/specs/pull/205, metadata is now an extension, and further extensions
will be added soon

Updates IPLD Bridge to use latest go-ipld-prime and runs and actual round trip test using GraphSync,
go-ipld-prime, and real selectors

Convert GraphSyncRequest and GraphSyncResponse to structs, track by value, pass directly into add
methods on GraphSyncMessage

Update to finalized protobuf format.
Also remove deprecated selector interface which is out of date

testutils will be general, so move them

Add ToNet and FromNet functions that read/write to an io stream for use in network. Also type
response codes

Use a Cid as a root instead of a node, so that only one round trip is neccesary

Initial implementation of graph sync messages including protobuf struct and
serialization/deserialization

fix #2 fix #3