multi: move Input interface and related code

This commit is a step to split the lnwallet package. It puts the Input
interface and implementations in a separate package along with all their
dependencies from lnwallet.

input/signer.go

@@ -0,0 +1,41 @@
+package input
+
+import (
+	"github.com/btcsuite/btcd/wire"
+)
+
+// Signer represents an abstract object capable of generating raw signatures as
+// well as full complete input scripts given a valid SignDescriptor and
+// transaction. This interface fully abstracts away signing paving the way for
+// Signer implementations such as hardware wallets, hardware tokens, HSM's, or
+// simply a regular wallet.
+type Signer interface {
+	// SignOutputRaw generates a signature for the passed transaction
+	// according to the data within the passed SignDescriptor.
+	//
+	// NOTE: The resulting signature should be void of a sighash byte.
+	SignOutputRaw(tx *wire.MsgTx, signDesc *SignDescriptor) ([]byte, error)
+
+	// ComputeInputScript generates a complete InputIndex for the passed
+	// transaction with the signature as defined within the passed
+	// SignDescriptor. This method should be capable of generating the
+	// proper input script for both regular p2wkh output and p2wkh outputs
+	// nested within a regular p2sh output.
+	//
+	// NOTE: This method will ignore any tweak parameters set within the
+	// passed SignDescriptor as it assumes a set of typical script
+	// templates (p2wkh, np2wkh, etc).
+	ComputeInputScript(tx *wire.MsgTx, signDesc *SignDescriptor) (*Script, error)
+}
+
+// Script represents any script inputs required to redeem a previous
+// output. This struct is used rather than just a witness, or scripSig in order
+// to accommodate nested p2sh which utilizes both types of input scripts.
+type Script struct {
+	// Witness is the full witness stack required to unlock this output.
+	Witness wire.TxWitness
+
+	// SigScript will only be populated if this is an input script sweeping
+	// a nested p2sh output.
+	SigScript []byte
+}