Strictly enforce the Commitments max size in CBOR (#835)

The cbor-gen does not seem to implicitly enforce the length specified in
the underlying Golang array type. Instead, it uses the maximum number
of allowed bytes if no `maxlen` struct tag is specified (2MiB).

Strictly enforce the max allowed size to avoid potential attack vector.

Relates to:
 * whyrusleeping/cbor-gen#106

gpbft/chain.go

@@ -56,7 +56,7 @@ type TipSet struct {
 	// Blake2b256-32 CID of the CBOR-encoded power table.
 	PowerTable cid.Cid
 	// Keccak256 root hash of the commitments merkle tree.
-	Commitments [32]byte
+	Commitments [32]byte `cborgen:"maxlen=32"`
 }
 
 // Validates a tipset.

gpbft/gpbft.go

@@ -85,7 +85,7 @@ type Justification struct {
 type SupplementalData struct {
 	// Merkle-tree of instance-specific commitments. Currently empty but this will eventually
 	// include things like snark-friendly power-table commitments.
-	Commitments [32]byte
+	Commitments [32]byte `cborgen:"maxlen=32"`
 	// The DagCBOR-blake2b256 CID of the power table used to validate the next instance, taking
 	// lookback into account.
 	PowerTable cid.Cid // []PowerEntry