Add MonadFix instance for Vector

[treeowl]

I believe this should be equivalent to the MonadFix instance for []:

instance MonadFix Vector where
  mfix f
    | V.null v0 = V.empty
    | otherwise = v
    where
      v = V.generate (V.length v0)
                     (\i -> if i == 0
                            then V.head v0
                            else f (v ! i) ! i)
      v0 = fix (f . V.head)

Of course, that generate can be replaced with something slightly more efficient, but I don't know if it's worth the trouble.

[treeowl]

Actually, it may be worth the trouble. We don't really want to hold on to v0 any longer than necessary. So we should likely walk over it monadically to pull the thunks out, using the MonadFix instance for ST s. I'm not completely sure that works, but I think it probably does.

[treeowl]

Actually, some of this attempt to avoid leaks may be useless (particularly the part that forces use of runST). I'll post an update and explanation.