Compare multihash `Code` to `IDENTITY` when extracting public key

This PR is a non-substantive change with the goal of making this method
`ExtractPublicKey` from a `peer.ID` clearer to the reader. This goal is
accomplished by using the full name of the identity hash code `IDENTITY` instead
of the deprecated `ID` code. This change makes its clearer to the reader that
there is a trivial way to compute the public key given the digest. Without this
change it is easy to confuse `ID` with a concept relating to peer IDs (and thus
implying this function somehow computes a hash pre-image to get the
corrresponding public key.

core/peer/peer.go

@@ -92,7 +92,7 @@ func (id ID) ExtractPublicKey() (ic.PubKey, error) {
 	if err != nil {
 		return nil, err
 	}
-	if decoded.Code != mh.ID {
+	if decoded.Code != mh.IDENTITY {
 		return nil, ErrNoPublicKey
 	}
 	pk, err := ic.UnmarshalPublicKey(decoded.Digest)