psbt: remove Creator struct, create New function as entry point into package

The creator struct really didn't do anything before, as a result in this
commit we move to get rid of it, and create a `New` method as
customarily used in go packages.

psbt/creator.go

@@ -4,44 +4,38 @@
 
 package psbt
 
-// The Creator has only one function:
-// takes a list of inputs and outputs and converts them to
-// the simplest Psbt, i.e. one with no data appended to inputs or outputs,
-// but only the raw unsigned transaction in the global section.
-
 import (
 	"github.com/btcsuite/btcd/wire"
 )
 
-// Creator holds a reference to a created Psbt struct.
-type Creator struct {
-	Cpsbt *Psbt
-}
+// MinTxVersion is the lowest transaction version that we'll permit.
+const MinTxVersion = 1
 
-// CreatePsbt , on provision of an input and output 'skeleton' for
-// the transaction, returns a Creator struct.
-// Note that we require OutPoints and not TxIn structs, as we will
-// only populate the txid:n information, *not* any scriptSig/witness
-// information. The values of nLockTime, nSequence (per input) and
-// transaction version (must be 1 of 2) must be specified here. Note
-// that the default nSequence value is wire.MaxTxInSequenceNum.
-func (c *Creator) CreatePsbt(inputs []*wire.OutPoint,
-	outputs []*wire.TxOut, Version int32, nLockTime uint32,
-	nSequences []uint32) error {
-	// Create the new struct; the input and output lists will be empty,
-	// the unsignedTx object must be constructed and serialized,
-	// and that serialization should be entered as the only entry for
-	// the globalKVPairs list.
+// New on provision of an input and output 'skeleton' for the transaction, a
+// new partially populated PBST packet. The populated packet will include the
+// unsigned transaction, and the set of known inputs and outputs contained
+// within the unsigned transaction.  The values of nLockTime, nSequence (per
+// input) and transaction version (must be 1 of 2) must be specified here. Note
+// that the default nSequence value is wire.MaxTxInSequenceNum.  Referencing
+// the PSBT BIP, this function serves the roles of teh Creator.
+func New(inputs []*wire.OutPoint,
+	outputs []*wire.TxOut, version int32, nLockTime uint32,
+	nSequences []uint32) (*Packet, error) {
 
-	// Check the version is a valid Bitcoin tx version; the nLockTime
-	// can be any valid uint32. There must be one sequence number per
-	// input.
-	if !(Version == 1 || Version == 2) || len(nSequences) != len(inputs) {
-		return ErrInvalidPsbtFormat
+	// Create the new struct; the input and output lists will be empty, the
+	// unsignedTx object must be constructed and serialized, and that
+	// serialization should be entered as the only entry for the
+	// globalKVPairs list.
+	//
+	// Ensure that the version of the transaction is greater then our
+	// minimum allowed transaction version. There must be one sequence
+	// number per input.
+	if version < MinTxVersion || len(nSequences) != len(inputs) {
+		return nil, ErrInvalidPsbtFormat
 	}
-	unsignedTx := wire.NewMsgTx(Version)
-	unsignedTx.LockTime = nLockTime
 
+	unsignedTx := wire.NewMsgTx(version)
+	unsignedTx.LockTime = nLockTime
 	for i, in := range inputs {
 		unsignedTx.AddTxIn(&wire.TxIn{
 			PreviousOutPoint: *in,
@@ -57,14 +51,13 @@ func (c *Creator) CreatePsbt(inputs []*wire.OutPoint,
 	// transaction; the unknown list can be nil.
 	pInputs := make([]PInput, len(unsignedTx.TxIn))
 	pOutputs := make([]POutput, len(unsignedTx.TxOut))
-	c.Cpsbt = &Psbt{
+
+	// This new Psbt is "raw" and contains no key-value fields, so sanity
+	// checking with c.Cpsbt.SanityCheck() is not required.
+	return &Packet{
 		UnsignedTx: unsignedTx,
 		Inputs:     pInputs,
 		Outputs:    pOutputs,
 		Unknowns:   nil,
-	}
-
-	// This new Psbt is "raw" and contains no key-value fields,
-	// so sanity checking with c.Cpsbt.SanityCheck() is not required.
-	return nil
+	}, nil
 }