Simplify Transaction constructor's options

[alcuadrado]

There are multiple ways to set the chain that a transaction belongs to, this can/should be simplified.

I'm opening this issue as a continuation of the discussions that happened here, here, and here.

[holgerd77]

Another question I have related to this: how justified is this constructor option split into "data" and "opts" dictionaries?

[alcuadrado]

I think they were separated because of how defineProperties work. But I don't have enough knowledge about the history of this library.

[holgerd77]

@alcuadrado ah ok, I think I already stumbled upon that one but forgot again, thanks!