Added a more restricve version, ParVec, which is not a transformer.

Rather, it's a single Vec state welded onto the LVish Par monad.
BUT this revision is exhibiting some VERY odd behavior.  I can compile or
interpret the ParVec.hs file directly, but when cabal attempts the compile
it gets this error:

    Control/LVish/ParVec.hs:123:15:
    Could not deduce (PC.FutContents (Par det s2) a)
      arising from a use of `PC.spawn_'
    from the context (Eq a)
      bound by the type signature for
                 forkWithVec :: Eq a =>
                                Int
                                -> (forall sl. ParVec sl elt det s2 a)
                                -> (forall sr. ParVec sr elt det s2 b)
                                -> ParVec s0 elt det s2 (a, b)