build/transaction_envelope.go
func (b *TransactionEnvelopeBuilder) Base64() (string, error) {
	bs, err := b.Bytes()
	return base64.StdEncoding.EncodeToString(bs), err
}
func (b *TransactionEnvelopeBuilder) Base64() (string, error) {
	bs, err := b.Bytes()
	return base64.StdEncoding.EncodeToString(bs), err
}
// Base64 returns a string which is the xdr-then-base64-encoded form
// of the builder's underlying transaction envelope