Prototype-based objects bug-fix

[Reuben Thomas <rrt@...>]

The definition of clone is wrong, because it doesn't keep the tag of the
object table; you can forget about curry and instead use

-- clone: Make a shallow copy of a table, including any tag
--   t: table
-- returns
--   u: copy of table
function clone(t)
  return merge(t, {})
end

-- 
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