How to ref value types in tables?

[cblake]

I'm not sure it has anything to do with `seq`-specialness as this also seems to 
work (as does changing `object` to `tuple` or `newTable` to `initTable`):
    
    
    import tables
    type Weight = object
      w: int
      labels: seq[string]
    var tab = newTable[string, Weight](8)
    let empty = Weight(w: 0, labels: newSeqOfCap[string](1))
    for token in stdin.lines:
      var wcell = addr tab.mgetOrPut(token, empty)
      wcell.w += 1; wcell.labels.add token
    echo tab
    
    
    Run