Rather than manually estimating an input's weight by adding up all the
involved components (fixed-size skeleton, compact-serialized lengths,
and the actual scriptSig / witness stack items) we can simply take use
of the serialization classes `CTxIn` / `CTxInWitness` instead, to
achieve the same with significantly less code.
The new helper is used in the functional tests rpc_psbt.py and
wallet_send.py, where the previous manual estimation code was
duplicated.