[StructUtil] add support that the Struct can nest any type to transform (#80)

* add support json tag attribute for StructToMap function

* add the structutil to provide more rich functions and fixed #77

* add support that the nested struct to map for structutil

* recover code

* add structutil unit test

* [StructUtil] add unit test

pointer/pointer_test.go

@@ -0,0 +1,18 @@
+package pointer
+
+import (
+	"github.com/duke-git/lancet/v2/internal"
+	"testing"
+)
+
+func TestExtractPointer(t *testing.T) {
+
+	assert := internal.NewAssert(t, "TestExtractPointer")
+
+	a := 1
+	b := &a
+	c := &b
+	d := &c
+
+	assert.Equal(1, ExtractPointer(d))
+}