Update to Replacement GMP page

Hello all, I made another update to the notes on Replacing GMP, at http:// hackage.haskell.org/trac/ghc/wiki/ReplacingGMPNotes . It's pretty basic and you'd probably find it shabby, but comments, modifications appreciated. I am still in the throes of trying to *portably* beat GMP for speed and accuracy. (The portability problem comes in when I may no longer rely on Altivec/SSE/3DNow! support on other processors and need to optimise in pure C.) -Pete