smartcontract: add Len to Builder

Which is useful in some cases.

pkg/smartcontract/builder.go

@@ -66,6 +66,13 @@ func (b *Builder) InvokeWithAssert(contract util.Uint160, method string, params
 	b.Assert()
 }
 
+// Len returns the current length of the script. It's useful to perform script
+// length checks (wrt transaction.MaxScriptLength limit) while building the
+// script.
+func (b *Builder) Len() int {
+	return b.bw.Len()
+}
+
 // Script return current script, you can't use Builder after invoking this method
 // unless you Reset it.
 func (b *Builder) Script() ([]byte, error) {

pkg/smartcontract/builder_test.go

@@ -0,0 +1,23 @@
+package smartcontract
+
+import (
+	"testing"
+
+	"github.com/nspcc-dev/neo-go/pkg/util"
+	"github.com/stretchr/testify/require"
+)
+
+func TestBuilder(t *testing.T) {
+	b := NewBuilder()
+	require.Equal(t, 0, b.Len())
+	b.InvokeMethod(util.Uint160{1, 2, 3}, "method")
+	require.Equal(t, 37, b.Len())
+	b.InvokeMethod(util.Uint160{1, 2, 3}, "transfer", util.Uint160{3, 2, 1}, util.Uint160{9, 8, 7}, 100500)
+	require.Equal(t, 126, b.Len())
+	s, err := b.Script()
+	require.NoError(t, err)
+	require.NotNil(t, s)
+	require.Equal(t, 126, len(s))
+	b.Reset()
+	require.Equal(t, 0, b.Len())
+}