A trust of real or personal property, or both, created as part of a stock bonus plan, pension plan, disability or death benefit plan, medical benefit plan, profit-sharing plan or retirement plan, for the exclusive benefit of employees or self-employed persons, to which contributions are made by an employer, or employees, or both, or by self-employed persons, for the purpose of distributing to such employees or self-employed persons the earnings or the principal, or both earnings and principal of the fund so held in trust, shall not be deemed to be invalid as violating any existing laws against perpetuities or suspension of the power of alienation of title to property; but such a trust may continue for such time as may be necessary to accomplish the purposes for which it may be created.