fix: produced array may have side-effect to given args

[jizhuozhi]

In some condition that given args has an array, for example:

	functions := map[string]ExpressionFunction{
		"func1": func(args ...interface{}) (interface{}, error) {
			return args, nil
		},
	}
	exp, _ := NewEvaluableExpressionWithFunctions("func1(args, 1, 2, 3)", functions)
	result, _ := exp.Evaluate(map[string]interface{}{
		"args": []interface{}{1, 2, 3},
	})

In general programming languages, it excepts []interface{}{[]interface{}{1, 2, 3}, 1.0, 2.0, 3.0}, but actually, it will return []interface{}{1, 2, 3, 1.0, 2.0, 3.0} with side-effects and may cause data race when multi-threads executing even expression is stateless.

The root cause is separatorStage using contextless left production rule, so no way to check the slice type left value is produced by separatorStage or user given parameter. There is a simple to check is using type alias to change the etype of produced slice.