Add `base32-string->bytevector' and `nix-base32-string->bytevector'.
* guix/utils.scm (bytevector-quintet-set!,
bytevector-quintet-set-right!, base32-string-unfold,
base32-string-unfold-right, make-base32-string->bytevector,
base32-string->bytevector, nix-base32-string->bytevector): New
procedures.
* tests/utils.scm ("base32-string->bytevector",
"nix-base32-string->bytevector"): New tests.
Fix `bytevector->nix-base32-string'.
* 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.