When creating a transaction one must know the the fee beforehand to set
appropriate amounts for outputs and to know the fee, weight is required.
So far we only had a method on an already-constructed transaction. This
method clearly wasn't helpful when constructing the transaction except
for hacks like temporarily adding an all-zeroes signature.
This change adds a function that can compute the transaction weight
without knowing individual bytes of the scripts, witnesses and other
elements. It only needs to know their sizes.
To make the API less error-prone a special, trivial, type is also added
for computing the lengths of witnesses.
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