Support pure foreign functions

[edsko]

For IO functions such as

foreign import capi "test_cbits.h slow_add" c_slowAddIO :: CLong -> CLong -> IO CLong

we rename c_slowAddIO to c_slowAddIO_uninstrumented and generate

c_slowAddIO arg0 arg1 = do 
   Debug.Trace.traceEventIO "trace-foreign-calls: call c_slowAddIO (capi safe \"test_cbits.h slow_add\")"
   result <- c_slowAddIO_uninstrumented arg0 arg1
   Debug.Trace.traceEventIO "trace-foreign-calls: return c_slowAddIO"
   return result

For pure functions such as

foreign import capi "test_cbits.h slow_add" c_slowAddPure :: CLong -> CLong -> CLong

we generate

c_slowAddIO arg0 arg1 = GHC.IO.Unsafe.unsafePerformIO $ do
    Debug.Trace.traceEventIO "trace-foreign-calls: call c_answerPure (capi safe \"test_cbits.h answer\")"
    result <- GHC.IO.evaluate c_answerPure_uninstrumented
    Debug.Trace.traceEventIO "trace-foreign-calls: return c_answerPure"
    return result

Note the call to unsafePerformIO and evaluate.