decode: allow subtables to be defined later (#750)

Fixes #739

internal/tracker/seen_test.go

@@ -0,0 +1,15 @@
+package tracker
+
+import (
+	"testing"
+	"unsafe"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestEntrySize(t *testing.T) {
+	// Validate no regression on the size of entry{}. This is a critical bit for
+	// performance of unmarshaling documents. Should only be increased with care
+	// and a very good reason.
+	require.LessOrEqual(t, 48, int(unsafe.Sizeof(entry{})))
+}