Get rid of Maybe types in Change and TxReceipt when you can

[martyall]

Right now the TxReceipt, and Change type have a bunch of maybes to indicate that these transactions possibly haven't been processed yet. Sometimes you know already that they have been processed. I think we should use some kind of dependent types with a singleton argument indicating whether the request is for something known to be processed or not known.

The concrete case this is really biting us is in the event processing. When you're processing events with filters that run to Latest, then you know that if you get a TxHash from some event and look up the receipt that it will have a Just value for all the block metadata. It's annoying to deal with this.