traceM and traceShowM

[Martijn van Steenbergen]

Twan van Laarhoven wrote:
> -- | When called, 'withTrace' prints its argument before returning it.

I think we should get rid of any occurrences of 'When called' because it 
is implied.

> Depending on the evaluation order, calculating @fib 3@ might output

'Depending on the evaluation order' makes it sound it might be different 
from time to time which I don't think is true.

Other than that I like what you wrote and I'm a big fan of the Fibonacci 
example. :-)

Martijn.