feat(sieve): add sieve module with Map

modules/sieve/example_test.go

@@ -0,0 +1,24 @@
+package sieve_test
+
+import (
+	"fmt"
+
+	"git.golem15.com/golem15/summercms/modules/sieve"
+)
+
+type post struct {
+	ID    int
+	Title string
+}
+
+func ExampleMap() {
+	posts := []post{
+		{ID: 1, Title: "Hello"},
+		{ID: 2, Title: "Summer"},
+		{ID: 3, Title: "Goodbye"},
+	}
+	// Pluck the titles with a field accessor.
+	titles := sieve.Map(posts, func(p post) string { return p.Title })
+	fmt.Println(titles)
+	// Output: [Hello Summer Goodbye]
+}