feat: add package maputil and Keys func

maputil/map.go

@@ -0,0 +1,16 @@
+// Copyright 2021 dudaodong@gmail.com. All rights reserved.
+// Use of this source code is governed by MIT license
+
+// Package maputil includes some functions to manipulate map.
+package maputil
+
+// Keys returns a slice of the map's keys
+func Keys[K comparable, V any](m map[K]V) []K {
+	keys := make([]K, len(m))
+
+	for k := range m {
+		keys = append(keys, k)
+	}
+
+	return keys
+}