* guix/utils.scm (gnu-triplet->nix-system): New procedure.
(%current-system): New variable.
* tests/utils.scm ("gnu-triplet->nix-system"): New test.
* tests/derivations.scm (%current-system): Remove. Update users to
use (%current-system) instead.
* guix/utils.scm (bytevector-quintet-ref-right,
bytevector-quintet-fold): New procedures.
(bytevector-quintet-fold-right): Add `quintet-fold' parameter; use it
instead of `bytevector-quintet-fold'.
(bytevector->base32-string): Pass BYTEVECTOR-QUINTET-FOLD as the
first parameter.
(bytevector->nix-base32-string): Pass BYTEVECTOR-QUINTET-FOLD-RIGHT as
the first parameter.
* tests/utils.scm ("sha256 & bytevector->nix-base32-string"): New test.