Added objects 'Has' and 'Keys' function (#69)

* add pkg/stdlib/objects Length function

* rename lenght.go -> length.go

* fix tests according to other tests

* add new tests to length tests

* delete objects method Length

* add objects method Has

* add objects function Keys

* small fixes in Keys and Has functions

* change Has function

* unit tests for Keys function

pkg/stdlib/objects/keys.go

@@ -0,0 +1,46 @@
+package objects
+
+import (
+	"context"
+	"sort"
+
+	"github.com/MontFerret/ferret/pkg/runtime/core"
+	"github.com/MontFerret/ferret/pkg/runtime/values"
+)
+
+func Keys(_ context.Context, args ...core.Value) (core.Value, error) {
+	err := core.ValidateArgs(args, 1, 2)
+	if err != nil {
+		return values.None, err
+	}
+
+	err = core.ValidateType(args[0], core.ObjectType)
+	if err != nil {
+		return values.None, err
+	}
+
+	obj := args[0].(*values.Object)
+	needSort := false
+
+	if len(args) == 2 {
+		err = core.ValidateType(args[1], core.BooleanType)
+		if err != nil {
+			return values.None, err
+		}
+
+		needSort = bool(args[1].(values.Boolean))
+	}
+
+	keys := sort.StringSlice(obj.Keys())
+	keysArray := values.NewArray(len(keys))
+
+	if needSort {
+		keys.Sort()
+	}
+
+	for _, key := range keys {
+		keysArray.Push(values.NewString(key))
+	}
+
+	return keysArray, nil
+}