sweep: create new Input interface

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.

utxonursery.go

@@ -871,14 +871,14 @@ func (u *utxoNursery) graduateClass(classHeight uint32) error {
 		// generated a sweep txn for this height. Generate one if there
 		// are kindergarten outputs or cltv crib outputs to be spent.
 		if len(kgtnOutputs) > 0 {
-			csvSpendableOutputs := make([]sweep.CsvSpendableOutput,
+			sweepInputs := make([]sweep.Input,
 				len(kgtnOutputs))
 			for i := range kgtnOutputs {
-				csvSpendableOutputs[i] = &kgtnOutputs[i]
+				sweepInputs[i] = &kgtnOutputs[i]
 			}
 
 			finalTx, err = u.cfg.Sweeper.CreateSweepTx(
-				csvSpendableOutputs, classHeight)
+				sweepInputs, classHeight)
 
 			if err != nil {
 				utxnLog.Errorf("Failed to create sweep txn at "+
@@ -1749,7 +1749,7 @@ func readTxOut(r io.Reader, txo *wire.TxOut) error {
 	return nil
 }
 
-// Compile-time constraint to ensure kidOutput and babyOutput implement the
-// CsvSpendableOutput interface.
-var _ sweep.CsvSpendableOutput = (*kidOutput)(nil)
-var _ sweep.CsvSpendableOutput = (*babyOutput)(nil)
+// Compile-time constraint to ensure kidOutput implements the
+// Input interface.
+
+var _ sweep.Input = (*kidOutput)(nil)