Proposal: add ifM and whenM to Control.Monad

[Simon Hengel]

> A quick heuristic grep over all Hackage packages results in quite a bit
> of packages containing the ifM/whenM/unlessM:

But that kind of shows that the "expected" names for those functions are
ifM/whenM/unlessM.  I would ask the question:

    Are there any other useful combinators that would be named
    ifM/whenM/unlessM under the current naming convention?

If no, then I'm not entirely convinced that we should decide against
what seems to be common intuition here.

In general, I'm not sure about ifM (as it does not line up with `bool`).

Cheers,
Simon