A hive key becomes a certificate subject and a directory name, but only the
reserved-name and fragment guards looked at it, so `a/b` or `../x` evaluated
and misbehaved later. Assert at eval time that every key is 1-63 characters
of [a-z0-9-], the rule `Ident` applies in hive-types, and name the offending
keys in the message.
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