That bonds issued under the provisions of this act [21-3-13, 21-3-14, 21-3-16 to 21- 3-28 NMSA 1978], and the income thereupon being for the sole purposes specified in Section 1 [21-3-13 N.M. Stat. Ann.] hereof, shall forever be and remain free and exempt from taxation by the state of New Mexico or any subdivision thereof.