library: reduce copies of items to minimum, <STAT> api

library: reduce copies of items to minimum, <STAT> api

This commit will consolidate the three separate copies
of the item enumerators currently supporting that reap
function (summary, cpus & nodes) into one shared copy.

That select function will continue to maintain its own
dedicated items copy, albeit via a new item structure.

[ and while we're at it, we'll move the 'select' guy ]
[ to its proper alphabetical place, after our 'reap' ]

Signed-off-by: Jim Warner <james.warner@comcast.net>