interp: take into account embedded property of struct field

The trick is that in reflect, the field is called Anonymous, which is
actually a different notion in the Go spec/vocable.

n.b. only works for non-recursive types for now.

Fixes #781

_test/struct56.go

@@ -0,0 +1,23 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+)
+
+type A struct {
+	IA InnerA
+}
+
+type InnerA struct {
+	Timestamp int64
+}
+
+func main() {
+	a := &A{}
+	b, _ := json.Marshal(a)
+	fmt.Println(string(b))
+}
+
+// Output:
+// {"IA":{"Timestamp":0}}