Helper to return the serialized transaction from a transaction object

[aaroncox]

A nice convenience method would be to add a .serialized get helper on to the Transaction structs. Currently we need to import the Serializer and serialize it ourselves using:

const serializedTransaction = Serializer.encode({ object: transaction }).array;

I'm sure there are more transaction transformation helpers that could also be useful on top of that. It's an area we might want to explore.