merkledag: change 'Node' to be an interface

Also change existing 'Node' type to 'ProtoNode' and use that most
everywhere for now. As we move forward with the integration we will try
and use the Node interface in more places that we're currently using
ProtoNode.

License: MIT
Signed-off-by: Jeromy <why@ipfs.io>

blocks.go

@@ -14,7 +14,6 @@ import (
 var ErrWrongHash = errors.New("data did not match given hash!")
 
 type Block interface {
-	Multihash() mh.Multihash
 	RawData() []byte
 	Cid() *cid.Cid
 	String() string