add intersection (#25)

Co-authored-by: Ruidy <rnemausat@newstore.com>

docs/content/collections/intersection.md

@@ -0,0 +1,24 @@
+---
+title: "Intersection"
+date: 2022-03-27T00:24:11-04:00
+---
+
+Intersection computes the list of values that are the intersection of all the slices.
+Each value in the result is present in each of the slices.
+
+```go
+package main
+
+
+import (
+	"fmt"
+	u "github.com/rjNemo/underscore"
+)
+
+func main(){
+    a := []int{1, 3, 5, 7, 9}
+    b := []int{2, 3, 5, 8, 0}
+
+    fmt.Println(u.Intersection(a, b)) // {3, 5}
+}
+```

intersection.go

@@ -0,0 +1,12 @@
+package underscore
+
+// Intersection computes the list of values that are the intersection of all the slices.
+// Each value in the result is present in each of the slices.
+func Intersection[T comparable](a, b []T) (res []T) {
+	for _, n := range a {
+		if Contains(b, n) {
+			res = append(res, n)
+		}
+	}
+	return res
+}

intersection_test.go

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+package underscore_test
+
+import (
+	"testing"
+
+	u "github.com/rjNemo/underscore"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestIntersection(t *testing.T) {
+	a := []int{1, 3, 5, 7, 9}
+	b := []int{2, 3, 5, 8, 0}
+	want := []int{3, 5}
+
+	assert.Equal(t, want, u.Intersection(a, b))
+}