unique function (#27)

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

docs/content/collections/unique.md

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+---
+title: "Unique"
+date: 2022-04-12T17:18:04-04:00
+---
+
+`Unique` returns a duplicate-free version of the slice. Only the first occurrence of each value is kept.
+
+```go
+package main
+
+import (
+	"fmt"
+	u "github.com/rjNemo/underscore"
+)
+
+func main() {
+	nums := []int{1, 4, 2, 5, 3, 1, 5, 2, 8, 9}
+	
+	fmt.Println(u.Unique(nums)) // 1, 4, 2, 5, 3, 8, 9
+}
+```

unique.go

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+package underscore
+
+func Unique[T comparable](values []T) (uniques []T) {
+	seen := make(map[T]bool, 0)
+	for _, v := range values {
+		if _, ok := seen[v]; !ok {
+			uniques = append(uniques, v)
+			seen[v] = true
+		}
+	}
+	return uniques
+}

unique_test.go

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+package underscore_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	u "github.com/rjNemo/underscore"
+)
+
+func TestUnique(t *testing.T) {
+	nums := []int{1, 4, 2, 5, 3, 1, 5, 2, 8, 9}
+	want := []int{1, 4, 2, 5, 3, 8, 9}
+
+	assert.Equal(t, want, u.Unique(nums))
+}