Unbounded Currency

The Currency type can no longer overflow. However, its MarshalSia method restricts the length prefix to one byte, giving a maximum value of 2^(255 * 8) =~ 10^614. Larger values can be used internally, but will fail to encode correctly.

Also, Currency arithmetic will always return a new value. This means that you often have to write sum = sum.Add(value) instead of just sum.Add(value), but it isn't a big deal. Plus, you can chain operations now, and they're 10x more readable. This change might have introduced some sneaky bugs. When I changed the return value from an error to a Currency, all the places where we had err = x.Add(y) threw compiler errors. But the places where we just had x.Add(y) were silent, so I had to hunt them all down manually. Hopefully I got them all.

Large-scale review of the consensus package is next.