Support nil pointers for Binary.

If the thing does not already have a typebyte declared,
a fake one will be given (0x01).
A TypeByte of 0x00 is reserved for nil things.
No nil-dogs.

types/tx.go

@@ -76,7 +76,7 @@ type TxInput struct {
 	Amount    uint64            // Must not exceed account balance
 	Sequence  uint              // Must be 1 greater than the last committed TxInput
 	Signature account.Signature // Depends on the PubKey type and the whole Tx
-	PubKey    account.PubKey    // Must not be nil, may be PubKeyNil.
+	PubKey    account.PubKey    // Must not be nil, may be nil
 }
 
 func (txIn *TxInput) ValidateBasic() error {