Valgrind support

This is based on code in gtk2hs.

$ valgrind -q ./finalizer_queue

finalizer_queue: internal error: stg_ap_v_ret
    (GHC version 6.10.3 for i386_unknown_linux)
    Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
Killed

Unfortunately this test doesn't crash without Valgrind.

My guess is that this bit is the problem:

    finalizer <- fixIO $ \dPtr -> mkThunk $ do
        freeHaskellFunPtr callback
        freeHaskellFunPtr dPtr

Perhaps the documentation should say not to do this?