This commit restores the sweep behavior of
filtering inputs with unknown witness types
from the final sweep transaction. Currently,
such inputs will be included without adding
anything to the weight estimate, possibly
creating a transaction with insufficient fees.
This commit introduces a common interface for sweep
inputs. It eliminates the type checking from UtxoSweeper.
Also the formerly present Amount() getter is removed. It was redundant
because this value is present in SignDesc().Output.Value as well.
