Pre-allocate array in Children() for mmap and add benchmark. (#133)

* Pre-allocate array in Children() for mmap and add benchmark.

* Fixed an append issue noticed by Mihai

* use append instead of direct assignment, they are equivalent but the
append is cleaner

---------

Co-authored-by: Steve McDaniel <[email protected]>
Co-authored-by: Ashley Jeffs <[email protected]>

gabs.go

@@ -267,7 +267,7 @@ func (g *Container) Children() []*Container {
 		return children
 	}
 	if mmap, ok := g.Data().(map[string]interface{}); ok {
-		children := []*Container{}
+		children := make([]*Container, 0, len(mmap))
 		for _, obj := range mmap {
 			children = append(children, &Container{obj})
 		}

gabs_test.go

@@ -1915,6 +1915,25 @@ func TestFlattenIncludeEmpty(t *testing.T) {
 	}
 }
 
+func BenchmarkChildren(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		jsonObj, err := ParseJSON(bigSample)
+		if err != nil {
+			b.Errorf("Error parsing json: %v\n", err)
+		}
+
+		_ = jsonObj.Children()
+
+		FOSI := jsonObj.S("firstOutter", "secondInner")
+		_ = FOSI.Children()
+		SOSI := jsonObj.S("secondOutter", "secondInner")
+		_ = SOSI.Children()
+		SOTI := jsonObj.S("secondOutter", "thirdInner")
+		_ = SOTI.Children()
+	}
+}
+
 func BenchmarkWildcardSearch(b *testing.B) {
 	sample := []byte(`{"test":[{"value":10},{"value":20}]}`)