txsize now just counts the serialized bytes

[JaredCorduan]

Our txsize function was written to make coin selection easier, but it turns out not to help due to witnesses changing during coin selection. So now it is both complex and not helpful. This PR simplifies it to just the actual size in bytes.

[dcoutts]

LGTM!

We'll also need to update the spec addendum.

[JaredCorduan]

I figured out what happened to the trace generation. We actually were making uses of the properties of our txsize function in the coin selection algorithm used by the transaction generators. We would check that our fee was large enough, and then swap out txouts.

Since the old txsize was actually helpful to us, I've renamed it as txsizeBound (and similarly minfeeBound) to be used by our generators. And perhaps others will find them helpful as well.