Use of general-purpose integers is often error-prone and annoying. We're
working towards improving it by introducing newtypes.
This adds newtypes for weight and fee rate to make fee computation
easier and more readable. Note however that this dosn't change the type
for individual parts of the transaction since computing the total weight
is not as simple as summing them up and we want to avoid such confusion.
Part of #630
Martin Habovstiak