It's easier for me to read `$` with whitespace around it.
Clean up LVarTraceScalable.hs and add Data.LVar.PairScalable and
Data.LVar.SetScalable:

  * The update to LVarTraceScalable.hs rips out any vestiges of wait
    lists that were left from when it was copy-pasted from
    LVarTraceIO.hs.

  * Data.PairScalable and Data.LVar.SetScalable are basically copied
    from Data.LVar.PairIO and Data.LVar.SetIO, though.  They "work"
    insofar as I can build and run test_scalable.exe (`make
    test_scalable.exe`).  But I didn't provide any wait list
    representation.
Update comments describing LVar representations.