New4/.../Natural.hs : Minor tweaks.
New3/ : Tweak safePlusWord and safeMinusWord.
New[12]/.../Type.hs : Grab plusNatural code from New3.

The plusNatural code in New3 and New4 was about 8 times faster than
the code in New1 and New2.
New*/ : Remove all uses of succ.

The succ function on Int includes overflow detection:

    ghci > succ (0x7ffffffffffffffe::Int)
    9223372036854775807
    ghci > succ (0x7fffffffffffffff::Int)
    *Exception: Prelude.Enum.succ{Int}: tried to take `succ' of maxBound

and we'd like to avoid that in performance critical code.