tax exemption.] The provisions of this act N.M. Stat. Ann. § 6-12-6 to 6-12-14 shall constitute an irrepealable contract with the holders of any of the bonds and coupons issued pursuant to this act, for the faithful performance of which the full faith and credit of the state of New Mexico is hereby pledged. The bonds issued under this act and the coupons thereto attached shall have all the qualities of negotiable paper under the Uniform Commercial Code [N.M. Stat. Ann. Chapter 55], except that bonds, with attached coupons, issued prior to the passage of that code shall have the qualities of negotiable paper under the existing law at the time of issue. Bonds issued under this act and the attached coupons shall not be invalid for any irregularity or defect in the proceedings for the issue and sale thereof, and shall be incontestable in the hands of bona fide purchasers or holders thereof for value. All bonds issued under the provisions of this act shall be exempt from taxation.