rfc43: add job-list 'ranks' constraint

Problem: A hostlist constraint operator is defined in the set of
supported job-list constraint operators, but there is no corresponding
ranks constraint.  The ranks constraint is useful not only in test
instances where multiple ranks may share the same hostname, but also
in cases where a specific rank is to be queried without dependence
on hostname (e.g. rank 0).

Define a supported `ranks` constraint operator in RFC 43.

spec_43.rst

@@ -212,6 +212,9 @@ based on the following constraint operators.
 ``hostlist``
     Designate one or more nodes in RFC 29 Hostlist format (*string*) and match jobs assigned to those nodes.  The job list module MAY limit the number of entries in a hostlist constraint to prevent long constraint match times.
 
+``ranks``
+    Designate one or more broker ranks in RFC 22 Idset form (*string*) and match jobs that were assigned to one or more of those ranks. The job list module MAY limit the number of entries in a ranks constraint to prevent long constraint match times.
+
 ``t_submit``, ``t_depend``, ``t_run``, ``t_cleanup``, ``t_inactive``
     Designate one timestamp with a REQUIRED prefixed comparison operator (*string*).  The accepted comparison operators SHALL be `>`, `<`, `>=`, and `<=`, for greater than, less than, greater than or equal, or less than or equal. A timestamp operator SHALL match jobs where the respective timestamp matches against the provided timestamp.