Revert "Update to the last Rust."
Revert "Update to the last Rust."

This reverts commit c1b91eefbb87437491de221af40245c3a4a855ff.

As a matter of fact, Bounded has not been deleted.
However it is no longer automatically exported by the stdlib.

sebcrozet/nalgebra,ProtectedMode/nalgebra,sebcrozet/nalgebra,mbStavola/nalgebra,tikue/nalgebra,tempbottle/nalgebra,jaredly/nalgebra,tomaka/nalgebra