* guix/pk-crypto.scm (token-string?): New procedure.
(canonical-sexp-nth-data): Return a symbol when the element is a
"token", and a bytevector otherwise.
(latin1-string->bytevector): Remove.
(hash-data->bytevector): Adjust accordingly.
* tests/pk-crypto.scm ("canonical-sexp-nth"): Adjust accordingly. Add
octet string example.
* guix/pk-crypto.scm: Rename procedures, variables, etc. from
'gcry-sexp' to 'canonical-sexp'. Add comment with references.
* guix/scripts/authenticate.scm, tests/pk-crypto.scm: Adjust
accordingly.