patch applied (ghc): Add history/trace functionality to the GHCi debugger

[Simon Marlow]

Thu May  3 06:19:55 PDT 2007  Simon Marlow <simonmar at microsoft.com>
  * Add history/trace functionality to the GHCi debugger
  
  The debugger can now log each step of the evaluation without actually
  stopping, keeping a history of the recent steps (currently 50).  When
  a (real) breakpoint is hit, you can examine previous steps in the
  history (and their free variables) using the :history, :back and
  :forward commands.

    M ./compiler/ghci/GhciMonad.hs -44 +7
    M ./compiler/ghci/InteractiveUI.hs -69 +146
    M ./compiler/main/GHC.hs -3 +7
    M ./compiler/main/InteractiveEval.hs -84 +167