Implement list and fix test

map.go

@@ -1,7 +1,6 @@
 package storage
 
 import (
-	"errors"
 	"fmt"
 )
 
@@ -30,7 +29,20 @@ func (m *Map) Close() error {
 }
 
 func (m *Map) List(ns []string, limits ...string) ([]string, error) {
-	return nil, errors.New("not impl")
+	namespace := resolveNamespace(ns)
+	limits = resolveLimits(limits)
+
+	keys := []string{}
+	if _, ok := (*m)[namespace]; !ok {
+		return nil, nil
+	}
+	for k := range (*m)[namespace] {
+		if k >= limits[0] && k <= limits[1] {
+			keys = append(keys, k)
+		}
+	}
+
+	return keys, nil
 }
 
 func (m *Map) Get(key string, ns ...string) ([]byte, error) {